Add the "Pay Day" move effect to battle, with unit tests

https://bulbapedia.bulbagarden.net/wiki/Pay_Day_(move)

> Pay Day does damage and scatters coins on the ground with a value equal to twice the user's level for each time it is used. Regardless of who used Pay Day, these coins are picked up afterwards if the player wins the battle. No money is obtained if they flee or if the player captures the opponent Pokémon.
