Maybe tweak %WrapForValidAsyncIteratorPrototype%.return along the lines of #197

[bakkot]

Ideally %WrapForValidAsyncIteratorPrototype%.return and %WrapForValidAsyncIteratorPrototype%.next should match as closely as possible. Right now, because of how AsyncIteratorClose is factored, %WrapForValidAsyncIteratorPrototype%.return can't just do a passthrough of the promise from the underlying iterator's .return (because its resulted is Await'd by AsyncIteratorClose). We should maybe not use AsyncIteratorClose and instead directly implement the logic so that we can pass through the promise directly, as we did in #197.

[bakkot]

If we do this change, we should make the corresponding change in the sync version, as in #205.