Replace incorrect "async iterable" token to "async_iterable"

[saschanaz]

(Will check the below boxes once we have consensus here)

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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