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)).

まず、Javaチェスエンジンのポーンの指し手処理と、駒および盤面のマスの表現を特定します。ポーンが8段目に到達したときのプロモーションを実装し、同じ色のビショップ、ナイト、ルーク、またはクイーンをサポートして、その指し手の間にポーンが置き換えられることを検証します。

索引モデルが issue の本文から書いたものです。