contains method
Returns true if the FormBloc contains fieldBloc
Implementation
bool contains(FieldBloc fieldBloc, {int? step, bool deep = true}) {
  final fieldBlocs = (flatFieldBlocs(step) ?? const []);
  if (deep) {
    return fieldBlocs.any((fb) => fb == fieldBloc);
  }
  return MultiFieldBloc.deepContains(fieldBlocs, fieldBloc);
}