This is a legacy of Godot 2 days before the inspector had support for groups.
"Properties" with a slash in their name can't be accessed from script unless
using `set()`/`get()` so they were not actual properties as far as script
languages are concerned.
Part of #17558.
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.
