This PR adds a `flake` function to haskell.nix projects. It can
be used to transform the outputs of project into a flattened structure
that can be used to make a `flake.nix` file for your project.
Because the nix code and commands used are different a lot of stuff in
the getting-started.md will not work with a `flake.nix`. So instead
of trying to add a flake section to that guide this PR adds a new
version for Nix Flake users.
