Add menu system and scene navigation

The changes include: - Moving script files to scripts/ directory -
Creating Start menu scene with Play/Quit buttons - Adding scene
switching between menu and gameplay - Creating theme for menu buttons -
Adding ESC key to return to menu

scripts/Paddle.gd

@@ -0,0 +1,25 @@
+extends CharacterBody2D
+
+const SPEED: float = 350.0
+
+@export var player_id: int
+@export var score_label: Label
+var score: int = 0
+
+func _input(event: InputEvent) -> void:
+  if event.is_action_pressed('up-%d' % player_id):
+    velocity = Vector2(0, -SPEED)
+  if event.is_action_pressed( 'down-%d' % player_id):
+    velocity = Vector2(0, SPEED)
+  if event.is_action_released('up-%d' % player_id) or \
+    event.is_action_released('down-%d' % player_id):
+    velocity = Vector2.ZERO
+
+func _physics_process(_delta: float) -> void:
+  move_and_slide()
+
+func increment_score():
+  # score one point
+  score += 1
+  # update the UI label
+  score_label.text = '%02d' % score