This CL implements collection of garbage in the shared spaces in
the shared heap isolate. GC on such an isolate should now work
correctly without worker isolates. Support for worker isolates will
be implemented in a subsequent CL.

Bug: v8:13267
Change-Id: I30125ce3b791e2faa0504d065f23639d6106e6b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3904647Reviewed-by: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83371}