Currently the ability to create shared isolates is partially exposed to
API. Instead of fully exposing it, this CL makes shared isolate and
shared heap handling transparent to the embedder.

If a flag that requires the shared heap is true (currently
--shared-string-table and --harmony-struct), the first isolate created
in the process will create and attach to a process-wide shared isolate.
Subsequent isolates will attach to that shared isolate. When that first isolate is deleted, the shared isolate is also deleted.

Bug: v8:12547
Change-Id: Idaf2947bc354066c44f2d10243e10162b1b7e4d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3848825Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Owners-Override: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82756}