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.
And fixups:
- Add missing bindings in RenderingServer
- Remove duplicate ArrayMesh enum bindings (they're in Mesh already)
- Remove redundant _unhandled_key_input binding in Control (it's in Node
already)
These classes have dozens of open bugs and missing features
which may not be fixed anytime soon.
It's probably better to document it upfront at this point.
