Add flake templates

[lilyball]

Is your feature request related to a problem? Please describe.
I'm finally checking out Nix flakes now that Nix 2.4 is out, and there's a bunch of extra stuff to remember. flake-utils looks really handy for things like setting up a devShell flake, but there's still annoying boilerplate there.

Describe the solution you'd like
It would be really handy if flake-utils could vend some templates for use with nix flake init. In particular I would love a template for devShell specifically that wraps shell.nix. The NixOS Wiki page on Flakes suggests

{
  description = "my project description";

  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let pkgs = nixpkgs.legacyPackages.${system}; in
        {
          devShell = import ./shell.nix { inherit pkgs; };
        }
      );
}

So having something like this as the template (perhaps with a better description though) would be very convenient.

[jessestricker]

I currently use flake-utils for my development shells and propose the following as a template in addition to one by @lilyball.
This one does not wrap legacy nix-shell environments and is a single self-contained file.

{
  description = "...";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, utils }:
    utils.lib.eachDefaultSystem (system: let
      pkgs = nixpkgs.legacyPackages.${system};
    in {
      devShells.default = pkgs.mkShell {
        packages = [ 
          # packages go here, like this:
          pkgs.zig
        ];

        # environment variables go here, like this:
        SOME_ENV_VAR = "42";
      };
    });
}

[zimbatm]

#63 exposes the examples as templates. Is that enough?

[jessestricker]

LGTM

[jessestricker]

Well I'm not the person who opened this issue. Maybe this should remain open until they acknowledge the solution.

[zimbatm]

For sure. I'll re-open if I hear an objection.