completions: for -e, suggest only slugs that are in the config.json file

[ee7]

Perhaps:

• if jq is installed, complete the slugs by parsing the track config.json file.
• otherwise, fall back to looking at directories in exercises/{concept,practice}. But note that configlet fmt|generate|sync -e foo won't work if foo isn't in the track config.json file.

[ee7]

I've thought of the smarter way to fix this, which avoids the dependency on jq.

Is there a program that can parse JSON, that we know is installed on every user's machine at configlet completion time? Yes. Configlet itself.

We could add a subcommand that we omit from the configlet --help message:

configlet __complete

such that tab completion of an exercise slug:

configlet sync --track-dir foo --exercise [Tab]

internally calls:

configlet __complete sync --track-dir foo --exercise

which parses the config.json file in foo and returns the appropriate slugs (for both Concept and Practice Exercises for fmt, and just Practice Exercises for sync).

Low priority for now.

[ErikSchierboom]

That is a much better option. Neat.