Per https://github.com/WebAssembly/gc/issues/234, this implements
"nominal" type definitions with explicit supertypes, and statically
typed RTT-less instructions for allocation and testing/casting.
This should be fully backwards compatible with existing Wasm modules.

Spec: https://bit.ly/3cWcm6Q ("version 4")

Bug: v8:7748
Change-Id: Id5a1399b368fdfad22036cfd66f1bef593e640f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3144916
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76844}