Builtin class methods with varcall are not available in GDExtension

[Bromeon]

The following methods on builtins take varargs and thus follow the varcall convention:

Callable::call
Callable::call_deferred
Callable::rpc
Callable::rpc_id
Callable::bind
Signal::emit

The variant_get_ptr_builtin_method API returns null when invoked for the respective variant types, methods names and hashes.

This likely needs an additional API on GDExtension side.

[Bromeon]

Related:

• Set vararg methods' ptrcall of builtin classes, and let them can be called without arguments. godotengine/godot#76047
• Implement vararg methods of builtin classes. godotengine/godot-cpp#1091

[Bromeon]

Looks like Godot implemented support for builtin methods via varcall, so we could provide them as well.