[v3] Add practices field to Practice Exercises
[v3] Add practices field to Practice Exercises

While working on adding prerequisites to the Practice Exercises, we found that we really need two keys for Practice Exercises:

- `practices`: Concepts that the Practice Exercise is actively helping a student to practice. These show up in the UI as "Practice this Concept in: TwoFer, Leap, etc`
- `prerequisites`: The actual Concepts that should have been learnt by a student in order for them be able to do that Exercise.

There are lots of advantages to having the two keys instead of using one key for two purposes, not least that having them split out gives the maintainers a lot more control over the order a student should be exposed to exercises (via the prerequisites) and also where they encourage them to try concepts (via practices), which in reality are quite different things.

We have updated [the docs](https://github.com/exercism/docs/blob/master/anatomy/tracks/practice-exercises.md#practices) with more details and explanation on this.

exercism/xlua