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/start.gd

@@ -0,0 +1,10 @@
+extends Control
+func _ready():
+  %Button_Play.pressed.connect(play)
+  %Button_Quit.pressed.connect(quit_game)
+
+func play():
+  get_tree().change_scene_to_file('res://scenes/gameplay.tscn')
+
+func quit_game():
+  get_tree().quit()