Add Grep exercise

[tofische]

Added a new Haskell practice exercise Grep based on the available problem specification.

[github-actions]

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[tofische]

Can I contribute?

[MatthijsBlom]

Sorry to keep you waiting. I haven't been doing great, and do not expect to in the near future, so I will probably not be terribly available.

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Currently, none of the exercises on the Haskell track truly deal with IO. I think this exercise would gain the track more if it did. It seems the ideal exercise for this, and some/most/all other tracks use it for this as well.