feat: generate declaration files

[danielroe]

Reference: https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API#getting-the-dts-from-a-javascript-file

Note that generating .d.ts files will dramatically increase the time required to run mkdist (from order of 50-100ms to 7-10s 500ms).

TODO:

• generates declarations for .vue files
• generates declarations for .ts files
• generates declarations for .js files
• add cli option
• use file cache rather than reading files from disk
• enable by default

Closes #2

[danielroe]

I've managed to improve performance but it's still 10x longer to emit types than just transpile the files. I can imagine enabling it when using siroc, but perhaps we leave disabled by default when running mkdist?

[pi0]

I've managed to improve performance but it's still 10x longer to emit types than just transpile the files. I can imagine enabling it when using siroc, but perhaps we leave disabled by default when running mkdist?

@danielroe I was going to benchmark as well. Yes disabling by default makes sense. We may be able to reuse same instance of typescript compiler across files and do it off-thread using a worker (for later)

[pi0]

Typescript compiler is anyway slow but performance should be fixed by 421b200. It think it would be nice we add support for vue-dts-gen too (or a vue-demi wrapper)