fix: drop the NUL byte from shortcutKey and add it to the default export

[giaBaoJS]

The NUL byte

src/index.tsx line 42 contained a literal U+0000 inside the template
literal that shortcutKey() builds its separator from, where a space was
clearly meant:

return `${item.id}<NUL>{${sortedData}}`;

It looks like a space in an editor. It is not one.

It reaches users. shortcutKey is documented public API
(docs/API.md), and yarn prepare copies the byte straight into
lib/module/index.js, which is main and the default export condition.
Every key handed back to a caller therefore carries a control character:
a problem for anything that logs it, JSON-encodes it, uses it as a React
key, or writes it to a store that rejects NUL bytes, such as a Postgres
text column.

It also made the entry point unreviewable. Git classified
src/index.tsx as binary (git ls-files --eol reports i/-text w/-text),
so any diff of the file rendered as Binary files differ, and git blame,
grep and GitHub code search all returned nothing for it. This PR's own
diff of that file still shows as binary because the parent blob is; it
renders as normal text from the next change onwards.

The fix is the single character that was intended.

shortcutKey missing from the default export

README.md states that every function is reachable both as a named export
and on the default export. shortcutKey was the only one that was not, so:

import AppShortcuts from '@giabaojs/react-native-app-shortcuts';
AppShortcuts.shortcutKey(item); // TypeError: not a function

Tests

Two added to src/__tests__/shortcuts.test.ts:

• the key holds no control characters, and equals a {"x":"1"};
• the default export carries every function the module exports by name,
  derived from the module itself rather than a hand-kept list, so a future
  export cannot drift out of it unnoticed.

Both were confirmed to fail against the unfixed source, each against only
its own fix, and to pass once restored. yarn lint, yarn typecheck,
yarn test and yarn prepare are all green, and the rebuilt
lib/module/index.js no longer contains the byte.