goForward method
Go forward one move in history
Implementation
String? goForward() {
  if (!canGoForward) return null;
  _currentIndex++;
  return _history[_currentIndex].fen;
}Go forward one move in history
String? goForward() {
  if (!canGoForward) return null;
  _currentIndex++;
  return _history[_currentIndex].fen;
}