This CL adds a new snapshot to hold objects that are in the shared heap
or may need to be in the shared heap depending on runtime flags.
Currently this is to support --shared-string-table, which puts all
in-place-internalizable strings, internalized strings, and the
string table into the shared heap.

The shared heap snapshot is never deserialized into client Isolates.
This means when V8 is started without a shared Isolate, the shared heap
snapshot is deserialized into all Isolates.

Bug: v8:12007
Change-Id: I7eeab73080cda2e8250a5a49747f25b2440a349d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3173905
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77309}