For entries in the string forwarding table, mark the internalized string
if the original string is marked.

The logic is moved from the string forwarding table implementation to
the mark compact implementation, using RootVisitor.

Bug: v8:12007
Change-Id: I860de75077c864dd4e5f2c47ab647d2eafcc5ced
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3610625Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80326}