Add line-up exercise

[BNAndras]

No description provided.

[BethanyG]

This looks great! Thank you so much! Only two things, I think at this point. Maybe three. Only one that is really actionable.

1. Do we need a hints file? On the fence here - this exercise is easy, but what is easy for you or me might not be for others. In any case, a hints.md can always be PRd later if you are so moved. I am happy either way.
2. I hate how long the test function names are. But that is just me whining. We pull from problem-specs, so it is what it is.
3. We don't have to move the exercise up next to strings and string-methods in config.json. At least not yet. I'll scratch that itch ... later..... 😂 .

Unless there is anything you'd like to change or add...I am going to push the big, green button in the next little while. 🚀