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/ball.gd

@@ -0,0 +1,31 @@
+extends CharacterBody2D
+
+const SPEED : float = 310.0
+
+@export var paddle_0 : Node2D
+@export var paddle_1 : Node2D
+
+
+func _ready() -> void:
+  initialize()
+
+func _physics_process(_delta: float) -> void:
+  var collision = move_and_collide(velocity * _delta)
+  if collision:
+    velocity = velocity.bounce(collision.get_normal())
+
+func initialize():
+  var extra_offset = 0.0 if randf() < 0.5 else PI
+  var angle = extra_offset + randf_range(-PI/3.0, PI/3.0)
+  velocity = Vector2(cos(angle), sin(angle)).normalized() * SPEED
+  position = get_viewport_rect().size / 2.0
+
+func _on_screen_exited() -> void:
+  #Update Score
+  if position.x <0.0:
+    paddle_1.increment_score()
+  else:
+    paddle_0.increment_score()
+  #Reint After 1 second
+  await get_tree().create_timer(1.0).timeout
+  initialize()