lint: add kebab-case checks

[ee7]

This commit implements some stricter checks for the track-level
config.json file.

configlet lint now checks that every value of every below key is a
"kebab-case string":

• slug
• exercises.concept[].slug
• exercises.concept[].concepts
• exercises.concept[].prerequisites
• exercises.practice[].slug
• exercises.practice[].practices
• exercises.practice[].prerequisites
• exercises.foregone
• concepts[].slug

We define a "kebab-case string" as a string that matches the regular
expression:
^[a-z0-9]+(-[a-z0-9]+)*$

See:

• https://github.com/exercism/docs/blob/4a42d3399139/building/configlet/lint.md#rule-configjson-file-is-valid
• https://github.com/exercism/docs/pull/113/files

---

This PR currently causes the following diff to the output of configlet lint:

rust

+The `exercises.concept.prerequisites` array contains `&str`, but every value must be lowercase and kebab-case:
+./config.json
+
+The `concepts.slug` value is `&str`, but it must be a lowercase and kebab-case string:
+./config.json
+
+Configlet detected at least one problem.
+For more information on resolving the problems, please see the documentation:
+https://github.com/exercism/docs/blob/main/building/configlet/lint.md

What should the definition of "kebab string" be?

• A string that only contains characters in {'a'..'z', '0'..'9', '-'} (the definition in e898324)
• A string that matches ^([a-z0-9]+)(-[a-z0-9]+)*$
• A string that does not contain a character in {'A'..'Z', '_', ' '}
• Something else

I think we can either

1. Disallow special characters for all kebab strings, and tell the Rust track to write the concept name as something like "string-slices" instead.
2. Allow special characters in a concept name, so something like &str is allowed, but disallow them in the other kebab strings.

See e.g.

• https://doc.rust-lang.org/std/str/index.html
• https://doc.rust-lang.org/book/ch04-03-slices.html?highlight=%26str#string-slices

[ErikSchierboom]

Option 1 is what we want. I'll update the docs to clarify.

[ErikSchierboom]

Love the extensive tests!