Add groupByNes

[kubukoz]

Sometimes it'd be nice to group a Set to non-empty Sets, like:

Set("a", "aa", "aaa", "bbb").groupByNes(_.length) <-> SortedMap(
  1 -> NonEmptySet.one("a"),
  2 -> NonEmptySet.one("aa"),
  3 -> NonEmptySet.of("aaa", "bbb")
)

I suggest that a SetSyntax trait is added with syntax for this, similar to the existing ListSyntax.
The way ListOps#groupByNel is implemented (list.toNel.groupBy(...))suggests that this could also involve having groupBy in NonEmptySet.

I'll gladly contribute that, given some pointers :)

[Obarros]

@kubukoz I can take a look on this one.

[kubukoz]

Sure, go for it :)

[kubukoz]

Closed in #2285