goToIndex method
Go to a specific position in history
Implementation
String goToIndex(int index) {
if (index < -1 || index >= _history.length) {
return _currentIndex >= 0 ? _history[_currentIndex].fen : _initialFen;
}
_currentIndex = index;
return _currentIndex >= 0 ? _history[_currentIndex].fen : _initialFen;
}