Replace incorrect "async iterable" token to "async_iterable"

[saschanaz]

This changes the async iterable grammar added by #1397 to async_iterable so that the grammar can stay within LL(1) rule.

Fixes #1490

• At least two implementers are interested (and none opposed):
  • Apple per annevk's approval
  • Gecko by me
• Tests are written and can be reviewed and commented upon at:
  • N/A (can't test without an actual API that use it)
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/356891478#comment2
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1911295#c1
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=277548#c2
  • Deno: async iterable should be converted to async_iterable denoland/deno#30020
  • Node.js: Implement support for async_iterable<T> WebIDL type nodejs/node#54175 (comment)
  • webidl2.js: feat: add support for parsing async_sequence<T> type w3c/webidl2.js#775 (comment) (the comment is enough for me as a maintainer)
  • widlparser: Add support for async_iterable<T> plinss/widlparser#90
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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[saschanaz]

And this caught a grammar problem:

 The WebIDL grammar is NOT LL(1) due to the following conflicts:
    1. PartialInterfaceMember:
       * "async" (54/58)

Seems we should change the grammar?

[annevk]

How widely deployed is this type? I suspect not very if at all? I wouldn't mind making it async_iterable instead if that would help.

cc @lucacasonato

[lucacasonato]

LGTM

[saschanaz]

Added comments to the same bugs filed by #1397 as most of them are still open.

[saschanaz]

widlparser merged the original PR and applied the grammar change, so bikeshed is good to go once they get the new dependency.

Do we have any other blockers here?