Add scenes, static scene-finding mechanism, and scene pool for visual hold/tap notes.

rhythm_game/note/visual/tap/tap_note.gd

@@ -0,0 +1,9 @@
+class_name TapNote extends NoteVisual
+
+var id: int = -1
+
+func reset() -> void:
+	id = -1
+
+func in_use() -> bool:
+	return id >= 0