In my first few months of using Nix, I used nix-shell extensively
(it is great!), but had a hard time remembering the shell.nix
syntax.

I'd often find myself starting out by using nix-shell with '-p', making the
command line grow longer and longer as I added additional '-p' parameters
for that project.

I think it would be helpful to have a '--save' option that, instead of starting
a nix shell, produces a shell.nix corresponding to the current set of parameters.
When passed with no parameters, it'd still give me a nice shell.nix to get
started with without having to hunt for an example.

This PR is of course a straw man, very incomplete and perhaps not the right
approach in the first place. I just wanted to provide something more than a
idea.

Would this be interesting to explore further? Are there any particular
requirements or desired behaviour that I should take into account?