Implement missing Node & Resource placeholders

Implemented by request of @neikeq to advance in the GDExtension version of Mono.

* If a Resource type is missing upon load, it will be remembered together with its data (Unless manually overriden).
* If a Node type is missing upon load, it will be also be remembered together with its data (unless deleted).

This feature makes working with GDExtension much easier, as it ensures that missing types no longer cause data loss.

editor/editor_node.cpp

@@ -5890,6 +5890,7 @@ EditorNode::EditorNode() {
 	SceneState::set_disable_placeholders(true);
 	ResourceLoader::clear_translation_remaps(); // Using no remaps if in editor.
 	ResourceLoader::clear_path_remaps();
+	ResourceLoader::set_create_missing_resources_if_class_unavailable(true);
 
 	Input *id = Input::get_singleton();