modules: add common

This should define modules that are identical, or very similar.

The driving force is to be able to use `my.profiles` on home-manager and NixOS
without repeating myself.

In the future I might migrate other modules, such as `nixos/system/nix`...

modules/common/default.nix

@@ -0,0 +1,24 @@
+# Modules that are common to various module systems
+# Usually with very small differences, if any, between them.
+{ lib, _class, ... }:
+let
+  allowedClass = [
+    "darwin"
+    "homeManager"
+    "nixos"
+  ];
+
+  allowedClassString = lib.concatStringSep ", " (builtins.map lib.escapeNixString allowedClass);
+in
+{
+  config = {
+    assertions = [
+      {
+        assertion = builtins.elem _class allowedClass;
+        message = ''
+          `_class` specialArgs must be one of ${allowedClassString}.
+        '';
+      }
+    ];
+  };
+}