Implement support for `async_iterable<T>` WebIDL type

[lucacasonato]

whatwg/webidl#1397 adds the async iterable<T> type to WebIDL (lifted from whatwg/streams).

WHATWG Streams are changing to use this API in whatwg/streams#1310.

[saschanaz]

Heads up that whatwg/webidl#1489 wants to change the grammar to async_iterable instead of async iterable. (And thus the title should change)

[tabatkins]

Further heads up: whatwg/webidl#1500 is changing the type to async_sequence<> instead (and the interface declaration to async_iterable<>, from async iterable<>).

[github-actions]

This issue has been marked as stale due to 210 days of inactivity.
It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.

[jazelly]

not stale, this is still valid