Add Float16Array

[bakkot]

The Float16Array proposal for JavaScript adds a new TypedArray type holding IEEE binary16 floats. This PR adds it to all of the lists of TypedArray types in WebIDL.

Implementations of the proposal are underway in at least Chrome and Firefox, though it'll be some time before they're shippable. I don't know what the appropriate time to merge this is but I figured I should at least open it.

This is a JS feature, but adding it to ArrayBufferView has implications for any API which takes a generic TypedArray, which is quite a few of them. Does that imply they all need new tests? Hopefully not?

Fixes https://github.com/whatwg/webidl/issues/1310.

• [x] At least two implementers are interested (and none opposed):
  • Chromium (implementing JS feature)
  • Gecko (implementing JS feature)
• [x] Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/pull/45483
• [x] Implementation bugs are filed:
  • Chromium: N/A, already implementing
  • Gecko: N/A, already implementing
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=257022
  • Deno: https://github.com/denoland/deno/issues/23277
  • Node.js: https://github.com/nodejs/node/issues/52416
  • webidl2.js: https://github.com/w3c/webidl2.js/pull/777
  • widlparser: https://github.com/plinss/widlparser/pull/86
• [x] MDN issue is filed: Seems safe to assume this will happen.
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff

[bakkot]

OK, opened a PR with tests.

[annevk]

Thanks @bakkot, I think the one thing remaining is that we should probably link the proposal if we are to merge this. Similar to how HTML has "[JSIMPORTATTRIBUTES]" and such. With that nit addressed I'd be willing to merge this Friday, barring any other feedback.

[bakkot]

I stuck a link to the proposal in the table which defines the types, but I'm not sure that's the best way to do it. Happy to take suggestions for alternative ways to include the link.

[annevk]

I think the placement is good, but you want to use something akin to https://github.com/whatwg/fetch/blob/4c3750d3277e3df5ad452f48314eb29ca43b6461/fetch.bs#L58-L115 to add an official looking reference which you'd then reference from there.

[bakkot]

Neat, done.