home-manager: set module class to "homeManager"

This enables a module system feature documented here:
https://nixos.org/manual/nixpkgs/stable/index.html#module-system-lib-evalModules-param-class

For example, it allows a mistake to be caught, which is loading a
NixOS module into home-manager. This only works when the offending
module declares what it's for with a `_class` attribute.

It is not expected that users declare the `_type`, because the payoff
is small. It is only expected to be set by generic code, such as
functions or libraries that help with the "publishing" of modules
(e.g. flake-parts, flake-utils).

The class feature has been available in the module system since
https://github.com/NixOS/nixpkgs/pull/197547, merged May 6, 2023. It
has been part of all releases since 23.05-beta. The last NixOS release
that did _not_ support it has been end-of-life for close to a year
now.

Example:

    (lib.homeManagerConfiguration {
      pkgs = nixpkgs.legacyPackages.x86_64-linux;
      modules = [{ _class = "nixos"; imports = [ ./foo.nix ]; }];
    }).activation-script

Corresponding error:

    error: The module <unknown-file> was imported into homeManager instead of nixos.

(`<unknown-file>` can be improved by also setting `_file`, if known; a
much older feature)

PR #5339

nixos/common.nix

@@ -13,6 +13,7 @@ let
 
   hmModule = types.submoduleWith {
     description = "Home Manager module";
+    class = "homeManager";
     specialArgs = {
       lib = extendedLib;
       osConfig = config;