Concept: *.Extra packages

# Design

## Goal

The Goal is to learn that the various *.Extra packages exist, and caveats to using them, and an idea of the things that they contain.

As discussed in [this issue](https://github.com/exercism/elm-test-runner/issues/48)

## Learning objectives

- Know that various .Extra packages exist, and might have useful time saving functions
- Know which *.Extra packages exist
- Know a little about the functions they contain

## Out of scope

- A complete knowledge of all the packages and the functions they contain (the package website is best for this)

## Concepts

The concept this exercise unlocks is:

- `extra-packages`

## Prerequisites

- lists
- dict
- maybe

## Analyzer

TBD depending on the exercise