Unlike the Stage 1 proposal, for simplicity the prototype does not add
any new syntax, instead opting for exposing a SharedStructType
constructor which takes an array of field names. This type constructor
returns constructors for shared structs.

Shared structs can be shared across Isolates, are fixed layout, have no
prototype, have no .constructor, and can only store primitives and
other shared structs.

The initial prototype does not have TurboFan support.

Bug: v8:12547
Change-Id: I23bdd819940b42139692bcdb53d372099b0d4426
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3390643Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79156}