add non-empty collection variants

[scarf005]

motivation:

it'd be nice to centralize nonempty collections into de-facto stdlib.

• https://typelevel.org/cats/datatypes/nel.html
• https://zio.dev/zio-prelude/functional-data-types/nonemptylist/
• https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/

example usage:

scala> List().head
java.util.NoSuchElementException: head of empty list
  at scala.collection.immutable.Nil$.head(List.scala:663)
  at scala.collection.immutable.Nil$.head(List.scala:662)
  ... 30 elided
                                                                                                                                                                                                                                        scala> List().headOption
val res1: Option[Nothing] = None

scala> NonEmptyList(1).head
1

[SethTisue]

one old discussion is at scala/collection-strawman#40 (probably there have been others)