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)).

Start by locating the Java chess engine's pawn-move handling and the representation of pieces and board squares. Implement promotion when a pawn reaches the eighth rank, supporting a same-color bishop, knight, rook, or queen, and verify that the pawn is replaced during that move.

Written by the indexing model from the issue text.