[WebIDL] Change async_iterable<> type to async_sequence<> and async iterable<> declaration to async_iterable<> 他

Remove async as a keyword
whatwg/webidl@da6f3b3

Do not throw if the attribute setter is called with no arguments
whatwg/webidl@909c3fe

Change async_iterable<> type to async_sequence<> and async iterable<> declaration to async_iterable<>
whatwg/webidl@a8b10f5

Author: triple-underscore