This adds a new type 'none' as part of the WASM GC MVP.
The type can only be used in combination with a nullable reference, e.g.
'ref.null none'.
A 'nullref' is implicitly convertible to any nullable reference type.

Bug: v8:7748
Change-Id: Ic5ab6cc27094b3c9103ce3584452daa34633612f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3755136
Auto-Submit: Matthias Liedtke <mliedtke@google.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81670}