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: exercism/docs#290

src/lint/track_config.nim

@@ -92,7 +92,7 @@ proc isValidPracticeExercise(data: JsonNode; context: string;
       hasString(data, "slug", path, context, maxLen = 255, checkIsKebab = true),
       hasString(data, "name", path, context, maxLen = 255),
       hasString(data, "uuid", path, context, checkIsUuid = true),
-      hasInteger(data, "difficulty", path, context, allowed = 0..10),
+      hasInteger(data, "difficulty", path, context, allowed = 1..10),
       hasArrayOfStrings(data, "practices", path, context,
                         allowedArrayLen = 0..int.high, checkIsKebab = true,
                         uniqueValues = true),