[Board] Implement enPassant move handling

Board.hpp

@@ -128,6 +128,7 @@ private:
     }
 
     void handlePawnDoubleAdvance(const Move &move, BitBoard bb, const Piece &movedPiece);
+    void handleEnPassant(const Move &move, const Piece &movedPiece);
 };
 
 std::ostream &operator<<(std::ostream &os, const Board &board);