To make an array from a string, we were passing an interior pointer to
the array's contents to NewStringFromUtf8 -- but the allocation could
cause that interior pointer to become invalid.

As part of the fix, also start to refactor to put the notion of UTF-8
variants closer to unibrow.  This will allow us to unify more
UTF-8/WTF-8/sloppy UTF-8 code paths.

Bug: v8:12868, v8:13028, v8:13036
Change-Id: I61f53a6882b5ebe1d0b38594412cb35ff0e28a94
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3742701
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81536}