lint(track_config): disallow a `difficulty` of `0` (#447)
lint(track_config): disallow a `difficulty` of `0` (#447)

With this commit, `configlet lint` now produces an error if there is a
Practice Exercise in the track-level `config.json` file that has a
`difficulty` value of `0`.

Typically, exercises with such a value were deprecated exercises.
However, a deprecated exercise still appears on the website for a user
who started that exercise before it was deprecated, and therefore an
exercise's difficulty level should not be changed when it is deprecated.

We have created a PR on every track that had at least one exercise with
a `difficulty` of `0`, updating them to use an allowed value.

See: https://github.com/exercism/docs/pull/290

exercism/configlet