Implement stalemate

If the king is not in check and no moves can be execute by the player (not only king moves, any piece moves) the game is finished with a draw. This situation is [stalemate](https://support.chess.com/article/682-what-is-stalemate).

The easiest way to test this is with a king in a corner and a opposite queen near him, in knight movement distance. That king is stalemated.

The issue names no files or tests, so start by tracing how the engine determines whether a king is in check, generates legal moves for all pieces, and ends the game. Done means a player with no legal move while not in check is declared drawn, including the described corner-king and nearby-queen position.

Written by the indexing model from the issue text.