Create inventory of concepts lacking exercises

Exercism v3 Practice Exercises (of which most are implementations of exercises in this repo) have a `practices` field.
This field list the concepts that can be practiced in that exercise.
What several maintainers found was that for concepts, there are very few exercises that can be used to practice that concept.
One example concept that comes to mind is `dates`, for which we have (out of the top of my head) just two exercises: `meetup` and `gigasecond`.
Contrast that with the number of string exercises (`anagram`, `isogram`, `pangram`, `bob`, the song-related exercises, and so on).

The idea of this issue is to somehow compile a list of those concepts we feel could benefit from having more exercises.
We can do this several ways:

1. Create a list with _all_ exercises and their concepts.
2. Create a list with the concepts with few (or no) exercises

The first option is a lot more work, but would have the benefit that it could be used as a reference for track maintainers when choosing an exercise to implement.
The second option is easier to do.

I'd be curious in hearing your opinion.