Generate C# singletons as non-static classes

[raulsntos]

Singleton classes are no longer generated as static classes so they can be inherited.

This fixes mono after #59140 (introduces a class PhysicsServer3DExtension that inherits from PhysicsServer3D which is a singleton class) and re-enables mono CI.

The disadvantage of this is we now have to access the singleton class using the Singleton property everytime, which is cumbersome, but it's consistent with how it works in the engine and godot-cpp (using the get_singleton method).

[raulsntos]

Engine.GetSingleton is still generated as static to avoid an infinite loop.

[neikeq]

The disadvantage of this is we now have to access the singleton class using the Singleton property everytime, which is cumbersome, but it's consistent with how it works in the engine and godot-cpp (using the get_singleton method).

This is precisely why I made them static classes. It's a pretty big downside. I'll see what can be done about PhysicsServer3DExtension.

[neikeq]

Another possible solution, if we can't find a different one, is to have a @GlobalScope-like class with static fields for each singleton. C# 10 introduces "implicit usings", which means we can add an implicit using static Godot.GlobalScope; and make it be exactly like GDScript. Although, we would need to move these classes to a different namespace, which won't be easy (Not just in the bindings generators, but other places that assume the namespace as well. It should be much easier once we merge the dotnet6 branch with master).

[neikeq]

For now, let's just ignore classes that inherit from singletons.

EDIT: We don't want to ignore all of those, as to not silence possible future regressions. Instead, let's hard-code a list of classes that should be ignored, with PhysicsServer3DExtension on it.

[raulsntos]

Superseeded by #59286