Initial commit. Implement music sync features.

resource_type/song.gd

@@ -0,0 +1,13 @@
+## Represents a level for a music audio that may contain multiple charts.
+class_name Song extends Resource
+
+var music: Music
+
+var charts: Dictionary[StringName, Chart] = {}
+
+
+func has_music() -> bool:
+	return music != null and music.stream != null
+
+func has_chart() -> bool:
+	return not charts.is_empty()