Saves a specific resource type to a file. The engine can save resources when you do it from the editor, or when you call `ResourceSaver.save(resource)`. This is accomplished with multiple `ResourceFormatSavers`, each handling its own format. By default, Godot saves resources as `.tres`, `.res` or another built-in format, but you can choose to create your own format by extending this class. You should give it a global class name with `class_name` for it to be registered. You may as well implement a [ResourceFormatLoader]. Gets the list of extensions for files this saver is able to write. Returns true if the given resource object can be saved by this saver. Saves the given resource object to a file. flags is a bitmask composed with FLAG_* constants defined in [ResourceSaver]. Returns OK on success, or an ERR_* constant listed in [@GlobalScope] if it failed.