Playground Templates

### Feature request

Having had the pleasure of tomsmeding on IRC providing me with a Free $ Free Monad $ Tutorial exercise, I think that, first, his exercises are excellent, second, the idea could get traction.

Haskell Playground might benefit from being able to set up templates, for both exercises, prototyping, and random play, so that users can generate a playground and mess around with code both for joy and learning.

This would also help the Haskell community a lot because when we deal with newbie questions, it might be useful to simply link them to an exercise book on Haskell Playground.

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There is, of course, the Codewars and Exercism alternative, but those have curation and thus make it more difficult to create exercises.

The issue names no files, tests, or implementation entry point. Start by surveying the Haskell Playground structure and clarifying whether templates are exercises, prototypes, or shareable playground setups; done would require an agreed template model and user flow.

Written by the indexing model from the issue text.