Implement damage calculation formula

To make Pokemon can combat with each other, it is first necessary to implement the damage calculation formula.
- References
  - https://bulbapedia.bulbagarden.net/wiki/Damage
  - https://www.trainertower.com/dawoblefets-damage-dissertation/
  - https://github.com/jake-white/VGC-Damage-Calculator
  - https://www.smogon.com/dp/articles/mechanics_level50