PGN move parsing

[mdab121]

I've added PGNMove struct (representable by string) as well as parsing PGN Moves.

You can now use this API as this: game.execute(move: "NF3") game.execute(move: "O-O") game.execute(move: "exd5") game.execute(move: "d8=Q+")

Parsing is one way for now – you can only apply text moves to a game.