This expands the existing mechanism for generic structs to also cover
abstract types. This involves:
- Moving the SpecializationKey from StructType to Type, so that it's
  also available to AbstractType.
- Moving the generic parameters out of the StructDeclaration AST node
  and using the existing GenericDeclaration AST node for generic structs
  and abstract types too.
- The GenericStructType declarable gets generalized to GenericType.

This will be useful for defining a Weak<T> type for weak pointers.

Bug: v8:7793
Change-Id: I183b3a038a143cf0ae5888150104c4a025fd736c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859623
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64533}