In preparation for the new `v`-flag, extend RegExp flags from 1-byte to
2-byte.

Bug: v8:11935
Change-Id: I2dacb5e8dba889947054aa5c155708b8d315b898
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3857561Reviewed-by: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82863}