Added reverse-string exercise

[cdimitroulas]

As part of preparation for 48in24, I saw in the Exercism forums that the reverse-string exercise will be coming up on the 23rd of January and the Haskell track did not have this exercise implemented.

This PR implements the exercise. Apologies if I have missed anything, this is my first time contributing to an Exercism language track.

[github-actions]

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[cdimitroulas]

Started a new topic in the forums for this: https://forum.exercism.org/t/added-reverse-string-exercise-to-haskell-track/9213

[ErikSchierboom]

@cdimitroulas Once the review suggestions are addressed, I'll merge this

[cdimitroulas]

@MatthijsBlom thanks for the review!

@ErikSchierboom did you see my comment on the forums about the example solution?

The example solution I included in the PR is a bit boring (it just uses Haskell’s reverse which feels like cheating 😅) so I’m open to rewriting that in another way such as using a recursive loop.

Curious to know how you folks feel about this?

[ErikSchierboom]

Curious to know how you folks feel about this?

The goal of the example solution is just to prove that there is a solution that passed the tests. In other words: to verify that students will be able to solve the exercise. So a simple example is perfectly fine.

[ErikSchierboom]

One final, tiny nit