LR-99: Battle Engine v1

Few notes:
* 20 is a magic number.
* Would compare experience to level (instead of the other way around). Logically, the two are equivalent. I would do this for readability / to make the statement more intuitive
* What if the player jumps 2 levels? The current logic assumes players will only step up one level at a time. (Even if you don't intend for player's experience to rise this quickly, the logic should be prepared to handle this possibility).