A Group used to group elements in this Iterable.
final iterable = [('a', 1), ('a', 2), ('b', 3)]; final aggregate = iterable.group.lists(by: (e) => e.$1); print(aggregate); // {'a': [('a', 1), ('a', 2)], 'b': [('b', 3)]}
@lazy @useResult Group<E> get group => Group._(this);