Creating importers?

Is anyone aware of an example of a GDExtension which adds an importer for a new file format? (I mean one that works like OBJ or glTF.)

I can't find an explicit description what each of the `ModuleInitializationLevel`s mean or what needs to be an `EditorSceneFormatImporter` vs. `EditorImportPlugin` vs. simply a `Resource` (or if those are even what I should be implementing?).

It's hard to use the built-in importers as examples because they are using the internal interface.

Any progress on some proper GDExtension docs somewhere? 😄