Support new WebIDL async_sequence<T> type #2480

Not immediately urgent given no current usage AFAIA, but will likely start to diffuse into definitions (whatwg/streams#1372 etc.)

In terms of binding, this maps to AsyncIterable<T> when converting from IDL to JS. When converting from JS to IDL, it can technically fall back on a sync iterator if an async iterator isn't found, so so would probably technically be something like AsyncIterable<T> | Iterable<T>.

This won't be an issue for the "primary" builds (ts6.0+), since the generator adds a <reference> to lib.2018.asynciterable, so Symbol.asyncIterator is always present in the project global scope.

This will be an issue for older targets, where there are no <reference> directives, and interfaces marked async_iterable have their iterator methods ([Symbol.asyncIterator]/entries/keys/values) separated out into the *.asynciterable definition files. If we follow the same logic, that is going to mean that any method or property that references async_sequence is going to have to be excluded from the main definition files. A change in approach might be necessary, ie. "backporting" the <reference> pattern to these older targets, or just typing async_sequences as any for older targets.