Added a level system with loading and transitions

Touching the goal blocks will now trigger a transition to the next level, and to test out this system, I developed two new levels for a total of three.

I decided to dynamically load level scenes by looking for scenes/levels/0.tscn, scenes/levels/1.tscn, etc. This system worked well, but when reaching the goal for the next level, it would try to load a nonexistent file and crash. I fixed this by calling a win() function instead of loading a level file when the file cannot be found.

Next, I plan to add a few more levels, or maybe work on some of the main UI. The newer levels will introduce other special blocks and mechanics.