Add support for linting typescript files

[gkjohnson]

As discussed in #16449 this PR adds support for linting the typescript definition files using the existing eslint rules. This means that Travis builds will fail if the typescript definition files in /src are not linted properly and the .d.ts files can be automatically fixed using npm run lint -- --fix.

In order to get VSCode to display lint errors inline I had to add "typescript" to the "eslint.validate" options like so:

"eslint.validate": [ "javascript", "javascriptreact", "html", "typescript" ]

The caveat here is that in d.ts files the linter seems to see the imported objects as "unused-variables" for some reason even when they're used in function signatures. For example:

If this is merged then --fix should be run and then the d.ts changes committed into the dev branch.

Edit: I've fixed the caveat

[gkjohnson]

If it's unclear the Travis CI is expected to fail until all the d.ts files are properly linted, which I think should happen by running npm run lint -- --fix after this is merged!

[mrdoob]

Thanks!

[mrdoob]

Looks good? 5cd59cb
I can see still some indentation inconsistencies... 🤔

[gkjohnson]

@mrdoob any specific places? I can look into a bit more when I get a chance -- maybe it's less clear through the Github UI.

[mrdoob]

If you search for OrthographicCamera.d.ts in that commit you'll see what I mean. Some constructors and comments get extra indent.

[gkjohnson]

Oh I see:

	/**
   * @param left Camera frustum left plane.
   * @param right Camera frustum right plane.
   * @param top Camera frustum top plane.
   * @param bottom Camera frustum bottom plane.
   * @param near Camera frustum near plane.
   * @param far Camera frustum far plane.
   */
	constructor(
    left: number,
    right: number,
    top: number,
    bottom: number

It looks like it's due to space / tab indent inconsistency. I'll have to look into why it didn't auto-fix those.