* Allows creating a GDExtension based 3D Physics Server (for Bullet, PhysX, etc. support)
* Some changes on native struct binding for PhysicsServer
This allows a 3D Physics server created entirely from GDExtension. Once it works, the idea is to port the 2D one to it.
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.
