Give compile-time error if registering a class without its own `_bind_methods()` function

[dsnopek]

It's already a compile-time error if you make a class without a _bind_methods() that descends from a native class (like Node, Control, etc).

But as pointed out on issue https://github.com/godotengine/godot-cpp/issues/1430, if you make a class that descends from another extension class, there won't be a compile-time error because the parent class's _bind_methods() will get picked up, and some things won't work correctly (ex virtual method registration, like _ready(), as mentioned in the issue).

This PR attempts to make it a compile-time error if the class doesn't have its own _bind_methods(), ignoring any _bind_methods() inherited from its parent class.

This seems to work in my testing on GCC!

Fixes https://github.com/godotengine/godot-cpp/issues/1430

[pupil1337]

I tested it, work good.

[dsnopek]

Thanks for the testing and review!~

[dsnopek]

Cherry-picked for 4.2 in PR https://github.com/godotengine/godot-cpp/pull/1465

[dsnopek]

Cherry-picked for 4.1 in PR https://github.com/godotengine/godot-cpp/pull/1466