Add Seq syntax parallel to ListOps

[nigredo-tori]

In ListOps we have a few helper methods for producing NonEmptyLists via grouping, emptiness check, etc.. We can do the same thing with Seq and NonEmptySeq.

My specific use case (which I have hit a few times by this point) is that I have a Seq[Value], and I want to group it into a Map[Key, NonEmptySeq[Value]]. At the moment there are two ways to do this:

val values: Seq[Value] = ???
def f(v: Value): Key = ???
// This switches to `List`s which can be inefficient. Also, this is noisy, and it has an "unsafe" call.
values.toList.groupByNel(f).mapValues(nel => NonEmptySeq.fromSeqUnsafe(nel.toList))
// This is still noisy
NonEmptySeq.fromSeq(values).map(_.groupByNem(f).toMap)(Map.empty)

This might be a subset of #4070, but that one is huge and stale. I'd rather have a separate issue for this specific pain point.