Add minimal devShell template

[eprovst]

This adds a minimal multi-architecture devShell template based on the approach that is now used for the trivial template (see #103).

I opted to list the inputs explicitly, unlike to the trivial template. In this form, when a name does not match an input, Nix tries to find it in the flake registry (thus resolving to the same commit as nix shell nixpkgs#hello). On flake based NixOS installs, the registry entry for nixpkgs is even set to the one used by the system, thus someone who wants to track the system nixpkgs can simply remove the corresponding input line from the template, which mimics the behaviour of nix-shell. edit: unfortunately, in a flake.nix file only the global registry is used, not the system nor user one (where that entry pointing to the system version lives).