This will allow us to more easily add a strict UTF-8 decoder, for use in
stringrefs.

Bug: v8:12868
Change-Id: I6835dca619417f4d2994d8283728cf8ebe599bd7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3714660Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81333}