Add a (stack?/nix?/?) template for the simple/advanced project starting points

I often want to start a new stacklock2nix project, but find myself copying-and-pasting from a previous example.

It would be convenient to use `stack new` and stack's templates ( https://github.com/commercialhaskell/stack-templates ) to get an starting point.

Trivia: It seems like stack-templates might not be accepting submissions though ( https://github.com/commercialhaskell/stack-templates/pull/121 ) so not sure if this is a dead-end.