feat: expose lib to allow easy composition

[yajo]

With this change, we can now use devshell like this:

{
  inputs = {
    devshell = {
      url = "github:numtide/devshell";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
  };

  outputs = {devshell, ...}: {
    devShells.x86_64-linux.default = devshell.lib.mkShell {};
  };
}

This makes it comfortable to use without flake parts. For instance, when composing with Blueprint it makes it easier to use a devshell.nix file without having to use overlays.