unable to generate queen side castling move

FEN : r1bq1rk1/pp2npbp/2np2p1/2p5/4P2P/2NPB1P1/PPP1QPB1/R3K2R w KQq - 0 1
Move generated for white king : {d2, f1, d1, g1}

Issue line: https://github.com/alexobviously/bishop/blob/9f9fe5461d4baf0f31f61892703b4290024b0f8d/lib/src/game/game.dart#L471

Following code seem to be fixing the issue

```dart
if (j != numMidSqs && from != midSq && board[midSq].isNotEmpty) {
// squares between to and from must be empty
valid = false;
break;
}
```