kingCastlesTo function
Returns the square the king moves to when castling.
Implementation
Square kingCastlesTo(Side side, CastlingSide cs) => switch (side) {
Side.white => switch (cs) {
CastlingSide.queen => Square.c1,
CastlingSide.king => Square.g1,
},
Side.black => switch (cs) {
CastlingSide.queen => Square.c8,
CastlingSide.king => Square.g8,
},
};