Feat: provide `nix shell` highlighting

[sambacha]

currently nix-shell does not have proper highlighting

bash

#!/usr/bin/env nix-shell
#!nix-shell -i bash -p nix-prefetch-git

nix-prefetch-git https://github.com/nixos/nixpkgs.git \
                 --rev refs/heads/nixos-unstable-small > ./nix/nixpkgs.json

nix

#!/usr/bin/env nix-shell
#!nix-shell -i bash -p nix-prefetch-git

nix-prefetch-git https://github.com/nixos/nixpkgs.git \
                 --rev refs/heads/nixos-unstable-small > ./nix/nixpkgs.json

console

#!/usr/bin/env nix-shell
#!nix-shell -i bash -p nix-prefetch-git

nix-prefetch-git https://github.com/nixos/nixpkgs.git \
                 --rev refs/heads/nixos-unstable-small > ./nix/nixpkgs.json