refactor(lib): add lib output to flake

This is needed so we can use subattributes of flake.lib

nix/lib/options.nix

@@ -0,0 +1,15 @@
+{ lib, ... }:
+let
+  inherit (lib.options) mkOption;
+  inherit (lib.types) str;
+in
+{
+  flake.lib.options = {
+    mkStrOption =
+      default:
+      mkOption {
+        inherit default;
+        type = str;
+      };
+  };
+}