This CL fixes 2 issues with string internalization when the string table
is shared:
1. In-place migration of a string's map to Internalized was done before
   it was sure that the string is going to be internalized (outside the
   critical section). To fix this problem StringTableKey::AsHandle() is
   now split into StringTableKey::PrepareForInsertion(), which is
   invoked outside the critical section and creates a copy if
   necessary, and StringTableKey::GetHandleForInsertion(), which is
   invoked inside the critical section only for string table misses.
   Migration of the map is handled by this method.
2. TryStringToIndexOrLookupExisting() didn't handle already internalized
   strings. So far this was impossible, as this method was only invoked
   for strings that were checked not to be internalized. However with
   a shared string table, the string could be internalized after the
   checks.

Bug: v8:12007
Change-Id: I193d6b54dc41360eee47d21cbcaa36d2652d85dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3368103Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78600}