Allow `--declaration` with `--allowJs`

[blakeembrey]

From #7535, it would be useful for JavaScript package authors to have the ability to generate declarations from their project. This would help with getting started on a type definition (it would be populated with known interfaces/functions/types that are exported) and would also allow the creation of continuous integration scripts that check the JavaScript types with the hand-coded definition for inconsistencies.

[mhegazy]

PRs are welcomed. the fix should be removing the error, making sure the call is wired correctly, adding tests and making sure the declaration emitter handles the JS types correctly,

[bengreenier]

anyone currently working on this? If not i may take a stab at it.

[mhegazy]

anyone currently working on this? If not i may take a stab at it.

got for it.

[endel]

This would be definitely helpful 👍

[trusktr]

👍