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.