This is a reland of 3cb4039c

Changes since revert:
- Fix FLAG_stress_scavenge interaction with shared Isolate
- Use the shared Isolate's global handles to keep shared values
  alive in transit during a postMessage

Original change's description:
> [string] Support shared strings in Value{Serializer,Deserializer}
>
> When FLAG_shared_string_table is true, postMessaging strings will share
> instead of copy.
>
> Note that not all operations on shared strings are supported, and shared
> strings may be slower than non-shared strings for some operations.
>
> Bug: v8:12007
> Change-Id: I3462128e15410d2568868143571571b3025722c1
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3277250
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Shu-yu Guo <syg@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#78614}

Bug: v8:12007
Change-Id: I5d9b99b2dac6f26d5ef046d7aec94f1a1d219419
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3389533Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78761}