toggle method
Toggles the bool value between false and true.
A shortcut for flag.value = !flag.value;
FIXME: why return this? fluent interface is not
 not really a dart thing since we have '..' operator
Implementation
// ignore: avoid_returning_this
Rx<bool?>? toggle() {
  if (_value != null) {
    subject.add(_value = !_value!);
    return this;
  }
  return null;
}