Exposes initial_async_iterator_prototype the same way that
initial_iterator_prototype is exposed, to let blink implement
async iterables better.

Bug: chromium:1087157
Change-Id: I94f65eedb3aad0bf62cb4808935f320fe4bf4e9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2300779
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68921}