Add `pawn promotion`.

Implement **Pawn Promotion**.

> **What is Pawn Promotion?**
> Promotion in chess is a rule that requires a pawn that reaches the eighth rank to be replaced by the player's choice of a bishop, knight, rook, or queen of the same color. The piece chosen cannot be another king nor another pawn. The new piece replaces the pawn on its starting square on the same move. The choice of the new piece is not limited to pieces previously captured, thus promotion can result in a player owning, for example, two or more queens despite starting the game with one. Know more from the wikipedia entry [here](https://en.wikipedia.org/wiki/Promotion_(chess)).

Empieza localizando el manejo de los movimientos de peón del motor de ajedrez en Java y la representación de las piezas y las casillas del tablero. Implementa la promoción cuando un peón alcanza la octava fila, admitiendo un alfil, caballo, torre o dama del mismo color, y verifica que el peón sea reemplazado durante ese movimiento.

Escrito por el modelo de indexación a partir del texto del issue.