Classes without ICU / Platform API dependency:
* Constants - String constants for options bag’s names and values,
valid option values, other constants.
* IntlUtils - Conversion between UTF-8 and UTF-16 ASCII strings,
convert string to bool, i.e. case- insensitive match against
“true”, lowercase ASCII strings.
* LocaleBCP47Object - Wrapper over a
BCP47Parser::ParsedLocaleIdentifier, provides factory method to
create instances from a BCP47 locale string, methods to get
canonicalized locale string and locale string without extensions,
and implements the CanonicalizeLocaleList() spec operation.
* OptionHelpers - Gets string, bool, and number option from options
bag.

ICU dependent classes:
* Collator - Intl.Collator implementation using ICU collator API.
* Locale - Conversion between BCP47 and ICU locale strings.
* LocaleResolver - Implements ResolveLocale() and SupportedLocales()
spec operations, supports both “lookup” and “best fit” locale matching.
“best fit” locale matching uses ICU acceptLanguage API.

Fix memory leaks in existing DateTimeFormat implementation in
PlatformIntlICU.cpp by wrapping created ICU object pointers in
unique_ptr.

Migrate ResolveLocale, SupportedLocalesOf, GetCanonicalLocales
API implementation to use the corresponding newly added supporting
classes.

Add test cases to verify invalid locales and options input would result
in throwing JS exception in collation.js. Add tests cases to verify
resolution of locale extensions and options in a new file
collation-resolved-options.js.

[Testing]

Build succeeds on Ubuntu 20.04. Run the following JS tests on Ubuntu
20.04, and all pass except for a few test cases in
date-time-format-apple.js due to locale data differences. The output
of running date-time-format-apple.js test is the same as before this
change.

```
$ ./build/bin/hermes ./hermes/test/hermes/intl/intl.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/get-canonical-locales.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/get-canonical-locales.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/collator.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/collator.js
$ LC_ALL=fr_FR _HERMES_TEST_LOCALE=fr_FR ./build/bin/hermes ./hermes/test/hermes/intl/collator-resolved-options.js
$ TZ=GMT ./build/bin/hermes ./hermes/test/hermes/intl/date-time-format-apple.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/date-time-format-apple.js
```

Run valgrind with above test runs and no memory leak detected.

Run Test262 tests for Collator, String-LocaleCompare, DateTimeFormat,
Date, and Intl. All non-skipped test cases pass.

```
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Collator/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/String/prototype/localeCompare/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/DateTimeFormat/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Date/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Intl/
```

Change type of string constants in Constants.h to char16_t[] from
std::u16string to avoid global constructors and unnecessary dynamic
allocations. Also, change structs to namespaces and specify inline
constexpr so they are compile-time constants and can be shared
across multiple .cpp files.

Move those newly added files for ICU-based implementation in
lib/Platform/Intl into lib/Platform/Intl/impl_icu/.

Review (const reference) std::(u16)string usages in function parameters
and change them to std::(u16)string_view if implementation does not need
a std::(u16)string object or a null-terminated C-style string, to avoid
unnecessary temporary std::(u16)string object construction.

[Testing]

Build succeeds on Ubuntu 20.04. Run the following JS tests on Ubuntu
20.04, and all pass except for a few test cases in
date-time-format-apple.js due to locale data differences. The output
of running date-time-format-apple.js test is the same as before this
change.

```
$ ./build/bin/hermes ./hermes/test/hermes/intl/intl.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/get-canonical-locales.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/get-canonical-locales.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/collator.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/collator.js
$ LC_ALL=fr_FR _HERMES_TEST_LOCALE=fr_FR ./build/bin/hermes ./hermes/test/hermes/intl/collator-resolved-options.js
$ TZ=GMT ./build/bin/hermes ./hermes/test/hermes/intl/date-time-format-apple.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/date-time-format-apple.js
```

Run valgrind with above test runs and no memory leak detected.

Run Test262 tests for Collator, String-LocaleCompare, DateTimeFormat,
Date, and Intl. All non-skipped test cases pass.

```
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Collator/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/String/prototype/localeCompare/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/DateTimeFormat/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Date/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Intl/
```

Change the cache of available locales to a function local static from
a static class member in LocaleResolver.

Turn classes with only static methods to namespaced functions.

Use arrays for small constant sets and arrays of pairs for small
constant maps.

[Testing]

Build succeeds on Ubuntu 20.04. Run the following JS tests on Ubuntu
20.04, and all pass except for a few test cases in
date-time-format-apple.js due to locale data differences. The output
of running date-time-format-apple.js test is the same as before this
change.

```
$ ./build/bin/hermes ./hermes/test/hermes/intl/intl.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/get-canonical-locales.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/get-canonical-locales.js
$ ./build/bin/hermes ./hermes/test/hermes/intl/collator.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/collator.js
$ LC_ALL=fr_FR _HERMES_TEST_LOCALE=fr_FR ./build/bin/hermes ./hermes/test/hermes/intl/collator-resolved-options.js
$ TZ=GMT ./build/bin/hermes ./hermes/test/hermes/intl/date-time-format-apple.js | ./build/bin/FileCheck --match-full-lines ./hermes/test/hermes/intl/date-time-format-apple.js
```

Run valgrind with above test runs and no memory leak detected.

Run Test262 tests for Collator, String-LocaleCompare, DateTimeFormat,
Date, and Intl. All non-skipped test cases pass.

```
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Collator/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/String/prototype/localeCompare/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/DateTimeFormat/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Date/
$ ./hermes/utils/testsuite/run_testsuite.py -b ./build/bin --test-intl -a ~/Downloads/test262/test/intl402/Intl/
```