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/gameplay.gd

@@ -0,0 +1,8 @@
+extends Control
+
+func _ready():
+    set_process(true)
+
+func _process(_delta):
+   if Input.is_action_pressed("key_exit"):
+      get_tree().change_scene_to_file("res://scenes/Start.tscn")