Implement en-peasant and casteling tracking in `GameState`

[Goju-Ryu]

Description
Implement a way for the GameState to keep track of the legality of castling and if en-peasant is an option

Suggestions

• use two boolean values in state to track legality of castling to either side and let ApplyMove change these as rules dictate.
• remember the last move performed to enable en-peasant check.
  • have delegate to check if any en-peasant is possible
  • let the last move be public so move generators can look up which piece is eligible for the move