[stringrefs] Implement string.new_wtf8_array

See https://github.com/WebAssembly/stringref/issues/1.

Bug: v8:12868
Change-Id: Ic1c9b55b1ec35c32c79d9b2cb0be243b96fc4453
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3734807


Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81444}