At the moment the shared heap is internally implemented as its own
isolate - the shared isolate. This CL prepares to remove the shared
isolate and replace it with shared spaces in the main isolate.

This CL introduces the --shared-space flag to opt-in into this shared
heap-approach. Isolate::is_shared_space_isolate() and
Isolate::shared_space_isolate() are added as well to identify the
main isolate (or shared space isolate).

Bug: v8:13267
Change-Id: I1a79c839de3b3b9cc988401e2e6e70ce3b02fa22
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3874928Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83050}