Add pawn attacking move at its diagonals.

Add an attacking or capturing move for Pawn.
> Unlike other pieces, the pawn does not capture in the same direction that it moves. A pawn captures diagonally forward one square to the left or right (see diagram).
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> Another unusual rule is the en passant capture. It can occur after a pawn advances two squares using its initial two-step move option, and the square passed over is attacked by an enemy pawn. The enemy pawn is entitled to capture the moved pawn "in passing" – as if it had advanced only one square. The capturing pawn moves to the square over which the moved pawn passed (see diagram), and the moved pawn is removed from the board. The option to capture the moved pawn en passant must be exercised on the move immediately following the double-step pawn advance, or it is lost for the remainder of the game.

For more info, check out the wikipedia entry [here](https://en.wikipedia.org/wiki/Pawn_(chess)#Capturing).

Issueではファイルやテストが指定されていないため、まずJava Pawnの指し手の実装と、既存の指し手生成テストを探してください。通常の斜め方向の駒取りを確認し、続いてen passantのルールと、それが直ちに失効することを確認してください。両方の取り方の動作が正しく表現され、既存の動作が維持されていれば完了です。

索引モデルが issue の本文から書いたものです。