Bug: v8:12868

A slight modification to the existing DFA-based UTF-8 allocator to allow
decoding surrogates, for use in decoding WTF-8.  We'll need to
additionally constrain the decoder to disallow surrogate pairs.

Change-Id: Ifddbf08d4eeeff8f270df52a68f01769ea790eec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3652787
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80654}