add target types for typescript files

[tdyas]

Add typescript_sources and typescript_source target types to represent TypeScript sources. Source glob should be *.ts.

[tdyas]

Open question: Should typescript files be included in javascript_sources targets?

NPM files may validly have a mixture of both .ts and .js files in the same directory.

• Typescript files must be transpiled into Javascript before they can be run though. Might be better to track the Typescript files in a separate target for ease of knowing what to transpile.
• On the other hand, this may be a case like the cc backend where C and C++ files just need a different compiler but still fall under the cc_sources target type.

[Eric-Arellano]

I suspect it'd be clearer to do typescript_sources. For example, it makes sense to ./pants check a TS source for type checking / transpilation, but not JS.

[chrisjrn]

I'm keen to get some sort of support for TypeScript working. I agree that a separate typescript_source type makes sense (eslint for instance needs some rules installed to make it work with TypeScript)

How do we feel about using codegen to emit JS sources as a general approach for running the tsc compiler?

[huonw]

With #20226 we now have support for these target types (and #20252 adds tailor support too), so this issue is fixed as written... but there's no check or fmt support. Potentially one can do something useful with them by hooking them up to package_json scripts? https://www.pantsbuild.org/2.20/reference/targets/package_json