copy method
Deep copy of the current Chess instance
Implementation
Chess copy() {
return Chess()
..board = List<Piece?>.from(board)
..kings = ColorMap<int>.clone(kings)
..turn = turn
..castling = ColorMap<int>.clone(castling)
..ep_square = ep_square
..half_moves = half_moves
..move_number = move_number
..history = List<State>.from(history)
..header = Map.from(header);
}