This CL unifies the fields for shared spaces for both the shared
isolate and the shared space isolate-approach. This allows to mostly
avoid separate code paths for both implementations.

While this CL already sets up everything needed for allocation with
--shared-space, allocation isn't fully working with this CL due to
other remaining issues.

Bug: v8:13267
Change-Id: Icdb40ed7045e33e6acbb97d3838fa374e6c24a2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3892786Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83280}