feat: Add sortOn to Data.List.NonEmpty

Adds `sortOn` to `Data.List.NonEmpty`, and adds
comments describing when to use it, compared to
`sortWith` or `sortBy . comparing`.

The aim is to smooth out the API between
`Data.List`, and `Data.List.NonEmpty`.

This change has been discussed in the
[clc issue](https://github.com/haskell/core-libraries-committee/issues/227).