Add commands for analyzing practice exercises and their prerequisites

To be able to add and change `prerequisites` and `practices` to practice exercises efficiently, it would be very helpful to have tools that do the following tasks:
- print all concepts that aren't required by any exercise, aren't practiced by any exercise, and maybe the difference between those two lists (concepts that are required, but aren't practiced)
- for a given concept, print all exercises that require it, that practice it, and maybe the difference between those two lists (exercises that require the concept, but don't practice it)

Start by locating configlet's command entry points and the data structures for exercise prerequisites and practices. Define commands for concept-level and exercise-level reports, including the requested differences, then verify the output against representative practice exercises and concepts.

Written by the indexing model from the issue text.