The reference types wasm proposal dropped all subtyping. Subsequently,
the 'anyref' type was renamed to externref.
This changes all references of the *type* anyref to externref.
Additionally, the flag that permits this extension is renamed to
"reftypes" to mirror the proposal name.

Bug: v8:7748
Change-Id: Icf323f13b9660fd10540e65125af053fca3a03f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2232941
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68270}