Implement note spawner.

rhythm_game/lane/lane.gd

@@ -4,6 +4,6 @@ class_name Lane extends Node2D
 func get_hit_pos() -> Vector2:
 	return position
 
-
-func update(_beat: BeatUpdate) -> void:
+## Should be connected to [signal NoteSpawner.notes_spawned].
+func update(beat: float) -> void:
 	pass