Add CI job to ensure practice exercise order

[mk-mxp]

Establish a reproducible ordering of exercises, see forum thread for details.

I chose sorting by difficulty directly, because that allows to use all 9 levels. And lower cased name did not sound bad to me.

Now someone would need to adjust difficulties to match some real difficulty 😃

[homersimpsons]

Thank you for the thread link. I have no opinion on sorting here, I'm not even sure why this sorting is used on the site versus sorting them consistently across tracks.

[mk-mxp]

If I remember correctly, the argument for track specific order was "Exercises might have different diffculties per track". As people like to solve exercises "in order", I hope they now solve more easy ones before attempting the hard ones.