let Object.get_class() work with custom types

[azur-wolve]

Currently can't use Object.get_class() for custom types:

String get_class() const

    Returns the object's class as a String. See also is_class().

    Note: get_class() does not take class_name declarations into account.
    If the object has a class_name defined, the base class name will be returned instead.

What about have a virtual method Object._get_class() to override and customize it?
Or even better, just let Godot return the name of the custom type as string.

[Calinou]

Duplicate of godotengine/godot#21789.