Add disposer and async_disposer

[jasnell]

An intial draft at adding the disposer and async_disposer keywords to declare that an interface implements Symbol.dispose and Symbol.asyncDispose:

interface Foo {
  disposer;
  async_disposer;
}

using foo = new Foo();
await using foo = new Foo();

• At least two implementers are interested (and none opposed):
  • Node.js
  • Cloudflare Workers
  • ...
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno: …
  • Node.js: …
  • webidl2.js: …
  • widlparser: …
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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

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[jasnell]

This is a first attempt at resolving #1459 /cc @annevk

[jasnell]

I'll wait to cleanup the remaining build errors once it is determined whether this will likely move forward or not.