Currently, developers need to manually remove editor plugins when their GDExtension is deinitialized, for example:

void uninitialize_summator_types(ModuleInitializationLevel p_level) {
	if (p_level == MODULE_INITIALIZATION_LEVEL_SCENE) {
		EditorPlugins::remove_by_type<MyEditorPlugin>();
	}
}

This isn't ideal, because:

1. Developers might forget to do this, leading to unexpected behavior in the editor
2. Developers don't need to manually unregister classes, so this is inconsistent with class registration
3. There is no EditorPlugins::remove_by_type<T>() in Godot, so this is inconsistent with how Godot modules work

This PR has godot-cpp automatically remove any editor plugins when the GDExtension is being uninitialized.

Since this functionality was added for Godot 4.1, I think it would be good to get this PR in for 4.1 as well, so that developers don't start using a pattern that is unnecessary.

This comes from a suggestion from @Zylann on #1114 (comment)