feat: add handling of non-flake features

[eureka-cpu]

Currently you cannot do ns nixpkgs#hello since it will think you are trying to reference a directory, we should make this a special variant and check if the user's system has flake capabilities, if not deferring to nix-shell instead.

[eureka-cpu]

We already partially handle this, when a flake.nix isn't present the shell.nix will be used, but in this case I'm referring to the commonality between nix shell and nix-shell. This should also handle the case where flake/nix-command features are not enabled.