Adds concurrent marking for reaching through v8::TracedReference.
Before this CL, a v8::TracedReference would always be processed on the
main thread by pushing a callback for each encountered reference.

This CL now wires up concurrent handling for such references. In particular:
- Global handles are already marked as well and not repurposed during
  the same GC cycle.
- Since global handles are not repurposed, it is enough to
  double-deref to the V8 object, checking for possible null pointers.
- The bitmap for global handle flags is mostly non-atomic, with the
  markbit being the exception.
- Finally, all state is wired up in CppHeap. Concurrent markers keep
  their own local worklist while the mutator marker directly pushes to
  the worklist owned by V8.

Bug: v8:12600
Change-Id: Ia67dbd18a57dbcccf4dfb9ccfdb9ee438d27fe71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3516255Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79736}