Add some extension methods

[natebosch]

This is technically breaking since it introduced the possibility for name conflicts, so we may as well consider them when we'll have a major version bump.

Some methods to consider (edit this comment to add more, we don't need to add them all):

• Iterable.firstWhereOrNull since it is no longer feasible to use Iterable.firstWhere(predicate, orElse: () => null);
• List.sortRange, List.shuffleRange - API Change Request: add List.sortRange and List.shuffleRange sdk#40841
• Iterable.enumerate - Addition to stdlib proposal: Iterable.enumerate sdk#32467
• Iterable<T?>.whereNotNull() returns Iterable<T>.

All existing top level methods should probably be moved to be extensions.

[lrhn]

I would really prefer to introduce a sublist functionality where you get a new fixed-length list backed by a slice of another list, then sortRange(range) and shuffleRange(range) would just be sublist(range).sort() and sublist(range).shuffle().
It's annoying to require every operation on lists to have to/from parameters.
(Ditto for strings, I'd love a lazy substring).

The rest are perfectly reasonable extension method suggestions.

[natebosch]

We can't use the name sublist, but we could use slice or range. I do like that idea over sortRange and shuffleRange.

[lrhn]

Extensions have landed