Optionally add relude and common GHC extensions

[utdemir]

relude is my go-to custom prelude, and I currently spend time adding the required mixins' section. Same with some GHC extensions.

I'd enable them unconditionally if this project were only for me, but it'd probably make other users happier if we prompt at the beginning.

[aycanirican]

Do we have any integration tests to ensure generated project is buildable?

[utdemir]

We do have a ci.nix which generates & builds the project with the default options; which is run by GitHub Actions. If we end up having more options, we should have more test cases for each combination of the options.

[chiroptical]

Putting this here for myself: https://hackage.haskell.org/package/relude-1.0.0.1#get-started

[utdemir]

Also, for reference, this is a hs-nix-template generated project that I modified to use relude and enabled some extensions: https://github.com/utdemir/nix-tree/blob/main/nix-tree.cabal