Allow creating editor plugins

[lilizoey]

In 4.1 it is now possible for gdextension libraries to register editor plugins. godotengine/godot#77010

It seems we just need to call editor_add_plugin to add a new plugin, and editor_remove_plugin to remove it. We should probably make the unregistering automatic by default. we can for instance add it in the default layer's deinit method. but we should make sure to document that if you override that function, you should either manually unregister editor plugins somewhere else, or re-add the automatic unregistration function.

[atty303]

I am writing EditorImportPlugin in Rust and would be happy to do this.

[Bromeon]

@atty303 could you elaborate your experience with it? Where would gdext come in handy?