Define UChar as char16_t

We used to have UChar defined as uint16_t which does not go along
with STL these days if you try to have an std::basic_string<> of it,
as there are no standard std::char_traits<> specialization for uint16_t.

This switches UChar to char16_t where practical, introducing a few
compatibility shims to keep CL size small, as (1) this would likely
have to be back-ported and (2) crdtp extensively uses uint16_t for
wide chars.

Bug: b:296390693
Change-Id: I66a32d8f0050915225b187de56896c26dd76163d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4789966
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Auto-Submit: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#89559}