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.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
ResourceFormatLoader and ResourceFormatSaver are meant to be overridden
to add support for different formats in ResourceLoader and ResourceSaver.
Those should be exposed as they can be overridden in plugins.
On the other hand, all predefined subclasses of those two base classes
are only meant to register support for new file and resource types, but
should not and cannot be used directly from script, so they should not
be exposed.
Also unexposed ResourceImporterOGGVorbis (and thus its base class
ResourceImporter) which are editor-only.
