Added the ability to select levels

Now you can find the Select level button in the main menu, and select the door you want to go to after entering the map. Add this feature because maybe my game is not fun enough right now? I want you to be able to play the game as much as you want, and I want to know if there's something wrong with my gameplay or what I'm doing right.
Next, I will try to find ways to make my game more interesting, and I really hope you can give me your comments or suggestions.