Bug: v8:12868

Also adds wtf8.cc, wtf8.h to src/wasm, to implement WTF-8 validation and
possibly other utilities.  Also fixes a bug when parsing the string
literals section; I had misunderstood the way the unordered/ordered
sections mechanism worked.

Change-Id: I3c4205e0872379a69575f84ba33e0090a9d8d656
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3652789
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80674}