This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
* Removed entirely from RenderingServer.
* Replaced by ImmediateMesh resource.
* ImmediateMesh replaces ImmediateGeometry, but could use more optimization in the future.
* Sprite3D and AnimatedSprite3D work again, ported from Godot 3.x (though a lot of work was needed to adapt them to Godot 4).
* RootMotionView works again.
* Polygon3D editor works again.
