Add haskell-simple-package

[harris-chris]

Hello - I found the haskell-hello template a little incomplete, so I've created this one - it's similar in structure, but:

• Has an executable, a library and a test defined, rather than just an executable
• Has a README
• Has its flake.nix written in (what I think is) a slightly more canonical format, using flake-utils

[tomberek]

Looks good. Requires updates to the root flake.nix to register the new template.

[harris-chris]

Sure. If there's any action for me then let me know.

[tomberek]

Yeah, there's a pattern in the flake.nix on how to add new ones.