Add nixngConfigurations option

Signed-off-by: Magic_RB <magic_rb@redalder.org>

flake.nix

@@ -39,6 +39,8 @@
   outputs = inputs@{ flake-parts, self, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } ({ config, ... }: {
       imports = [
+        modules/nixngConfigurations.nix
+
         nixos/systems/omen
         nixos/systems/heater
         nixos/systems/toothpick

modules/nixngConfigurations.nix

@@ -0,0 +1,29 @@
+# copied straight from https://github.com/hercules-ci/flake-parts/blob/main/modules/nixosConfigurations.nix
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    literalExpression
+    ;
+  inherit (flake-parts-lib)
+    mkSubmoduleOptions
+    ;
+in
+{
+  options = {
+    flake = mkSubmoduleOptions {
+      nixngConfigurations = mkOption {
+        type = types.lazyAttrsOf types.raw;
+        default = { };
+        description = ''
+          Instantiated NixNG configurations.
+          `nixngConfigurations` is for specific machines. If you want to expose
+          reusable configurations, add them to [`nixosModules`](#opt-flake.nixosModules)
+          in the form of modules (no `lib.nixosSystem`), so that you can reference
+          them in this or another flake's `nixngConfigurations`.
+        '';
+      };
+    };
+  };
+}