itemCanCheck method
检查某个子是否能将军
Implementation
bool itemCanCheck(ChessPos pos, int team) {
ChessPos? kPos = fen.find(team == 0 ? 'k' : 'K');
if (kPos == null) return true;
List<String> points = movePoints(pos, kPos);
return points.any((point) {
ChessFen newFen = fen.copy();
newFen.move(pos.toCode() + point);
return ChessRule(newFen).isCheck(team == 0 ? 1 : 0);
});
}