With this CL shared strings can be externalized and external strings can
be shared.
The StringForwardingTable is used to delay the real transition to the
next full GC. On the API side strings marked for externalization will
look like externalized strings.

Bug: v8:12957
Change-Id: I53b6509129bc5679c06bdf99421bdb41ea5d9082
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3849643Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82966}