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

rhythm_game/note/visual/note_visual.gd

@@ -0,0 +1,10 @@
+@abstract class_name NoteVisual extends Node2D
+
+enum TYPE {
+	TAP = 0,
+	HOLD = 1
+}
+
+@abstract func reset() -> void
+
+@abstract func in_use() -> bool