Add public API .d.ts file

[elalish]

Fixes #1502

It does generate the single file, however it is giant and includes much more than our public API. I'll consider this blocked on Swatinem/rollup-plugin-dts#133 unless a better way can be found.

[michaelkochhugobosscom]

any news about that ?

[elalish]

No; can you give some detail of your desired dev process and what you're hoping to do? Most of our users just put MV in as HTML; it'll be helpful to know what TS users are up to. Our unbundled library is also published on npm, so it should be possible to import the files from /lib instead of the bundle, since those all have .d.ts files associated. I'd still like to get a single d.ts file that only includes our public API, but I have yet to figure out a good way to do that.

[Ducki]

'[…] and what you're hoping to do'
Well, for a start: not getting never ending errors and compilation failures because Typescript doesn't understand what I'm importing and using.
Further: actually using Typescript with the API of the model-viewer (also without getting errors and failing to compile).

[elalish]

Closing in favor of #3718