Bug: v8:12868

Also adds the equivalent of Utf8Decoder, but for WTF-8.

Change-Id: I1548a44b0aea912cdd429eb85be4dfc606355cad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3660257Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andy Wingo <wingo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#80750}