Move enum registration from ClassDB / ClassInfo to GDType.

[Ivorforce]

• Follow-up of Expand GDType to cover GDExtension types as well #106016
• Works towards Core: Add first-class Object types, replacing (most of) ClassDB godot-proposals#12323

Now it's starting to get interesting :)
This PR is the first step towards replacing ClassDB (for the purposes of type capabilities), as proposed in godotengine/godot-proposals#12323.

Explanation

The PR makes a few important decisions:

• GDType gets init_state which protects against changes after the type gets its first subtype. This should be compatible with all GDExtensions, at least if they use the extension APIs as expected (with something like Godot's own _bind_methods called immediately after registration).
• constant_map and enum_map are cached (like name_hierarchy) from constants and enums across all supertypes. This avoids the expensive walk through parents on lookup that ClassDB is doing in master.
  • self_constant_map and self_enum_map keep track of the declarations on the type itself, which is also sometimes relevant.
• All maps are exposed as their actual type (const AHashMap &). ClassDB opted for a more verbose function-based interface, which I think is reinventing the wheel for interacting with hash maps. Internally, exposing the hash maps should give us more powerful tools for introspection.
• EnumInfo is exposed, for similar reasons.
• initialize_class is slightly better protected against multi-threaded shenanigans (with BinaryMutex). We don't expect more threads to access it, but it won't hurt to be sure about it.
• constant_order is removed. It only existed in DEBUG (making the builds differ in behavior) and was duplicating entries in the return List.