50 move rule

[blairjordan]

fide rulebook (https://www.fide.com/fide/handbook.html?id=171&view=article):

9.3 | The game is drawn, upon a correct claim by a player having the move, if: he writes his move, which cannot be changed, on his scoresheet and declares to the arbiter his intention to make this move which will result in the last 50 moves by each player having been made without the movement of any pawn and without any capture, or the last 50 moves by each player have been completed without the movement of any pawn and without any capture.

[blairjordan]

Note: need to include this in FEN input/ output.

Current FEN input/output doesn't include castling availability, which comes before halfmove clock, so need to either complete this first, or add a placeholder in the output.