The JSRegExp heap object should not be the source of truth for regexp
flags, which are also relevant in places that don't need or want to
care about the heap object layout (e.g.: the regexp parser).

Introduce RegExpFlags as a new source of truth, and base everything
else on these flags.

As a first change, remove the js-regexp.h dependency from the regexp
parser. Other files in src/regexp/ should be updated in follow-up
work.

Change-Id: Id9a6706c7f09e93f743b08b647b211d0cb0b9c76
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3103306Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76379}