Implement end game detection logic for "chess" sample project

[TAGC]

I've put together a basic chess console application to provide a practical demonstration of how this library can be used. Currently, the end game detection logic in Game.cs is placeholder, and it would be a good idea to implement the logic for that.

Specifically, Game.CheckGameOver(Board board, out Player? winner) should:

• return false and set winner to null if the game is not yet over
• return true and set winner to the winning player (black or white) if board represents a state in which one player has checkmated another
• return true and set winner to null if the board represents a stalemate

Ordinarily there would be test cases in place to validate the implementation. However, as this is an example project there are no test cases, as it's only used for demonstration purposes so it's not critical if there are a few bugs in it.

[TAGC]

I'll also need to update the game Complete logic to account for stalemates (currently it will only declare a game is complete if Winner is not null).

[TAGC]

Additional clarification:

• A player ('A') has checkmated another player ('B') if B's king is checked and there's no move available to B to bring the king out of check.

Also, for the time being stalemate logic can be omitted.