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

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.