Add String.concat on List.repeat call simplifications

[lue-bird]

Adds the last String simplification from #2

String.concat (List.repeat n str)
--> String.repeat n str

including composition.

Bonus: Corrected test names.

I'm thinking we could add the same for String.concat (List.intersperse s strings) --> String.join s strings

[github-actions[bot]]

The branch can be tried out by running:

elm-review --template jfmengels/elm-review-simplify/preview

[jfmengels]

I'm thinking we could add the same for String.concat (List.intersperse s strings) --> String.join s strings

That sounds good :+1: I'll add it to #2

[lue-bird]

Nice! I imagine the intent is to have these new utilities make it to our other existing simplifications?

Exactly right.