feat: add top-level lib option

For parity with NixOS:

https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/misc/lib.nix

modules/misc/lib.nix

@@ -0,0 +1,15 @@
+{ lib, ... }:
+
+{
+  options = {
+    lib = lib.mkOption {
+      default = { };
+
+      type = lib.types.attrsOf lib.types.attrs;
+
+      description = ''
+        This option allows modules to define helper functions, constants, etc.
+      '';
+    };
+  };
+}

modules/module-list.nix

@@ -1,6 +1,7 @@
 [
   ./alias.nix
   ./documentation
+  ./misc/lib.nix
   ./security/pki
   ./security/sandbox
   ./system