Add deno transpile subcommand

[bartlomieju]

Add deno transpile subcommand to transpile TypeScript to JavaScript and (maybe) generate type declaration files.

It should NOT translate Deno APIs to Node.js APIs, essentially a dumb transpilation thing.

deno transpile --out=dist/ --declaration ./webpage

Some thoughts about what should be supported:

• Take a bunch of .ts files (a directory, or a glob) and spit out a directory of .js files + .d.ts files
• Rewrite .ts extensions in import statements into .js extensions, because TSC refuses to transpile/generate declaration files if there are .ts extensions. NOTE: this is something TSC explicitly doesn’t do (ie. rewrite file extensions)
• Add a flag to set transpilation target (ECMA version) or respect compilerOptions.target from deno.json
• Add a flag to specify an output directory (—out), if not passed use out/ maybe?
• Add a flag to generate declarations (—declarations)
• Add a flag to generate source maps (—sourcemap),
• Blindly rewrite .ts , .jsx and .tsx imports to .js, but don’t rewrite .d.ts imports (???)

[BlackAsLight]

This seems like a weaker version of what deno bundle did