Sort exercises by difficulty

First of all, thank you for your great work! :green_heart:

I just started Unison a week ago as part of Mindshifting May. For my first few exercises, I was having a hard time finding the most appropriate one to tackle next.

I opened a post on the forum a few days ago about to suggest a feature for the Exercism website, the [possibility to sort exercises by validation number/rate](https://forum.exercism.org/t/feature-idea-sort-exercises-by-number-of-validation/5849). There, I learned by Jeremy that [the order is set manually by maintainers](https://forum.exercism.org/t/feature-idea-sort-exercises-by-number-of-validation/5849/6).

After taking a look at [the track's config history](https://github.githistory.xyz/exercism/unison/blob/main/config.json), I saw exercises here were first sorted alphabetically. It would be great if they could be sorted by difficulty instead.

I see the difficulty on a scale of 10 is also indicated. I would be glad to open a PR to sort them according to this. However, I may not be able to be more precise for exercise with the same difficulty value.

Cheers :wave:

Start with config.json and review its history to understand how the exercise order is represented. Check the listed difficulty values and determine how equal-difficulty exercises should be handled. Done means the exercise ordering reflects difficulty consistently in config.json.

Written by the indexing model from the issue text.