nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/hardware/mx-ergo/default.nix

@@ -0,0 +1,26 @@
+# Hold down the `next page` button to scroll using the ball
+{ config, lib, ... }:
+let
+  cfg = config.my.hardware.mx-ergo;
+in
+{
+  options.my.hardware.mx-ergo = with lib; {
+    enable = mkEnableOption "MX Ergo configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.xserver = {
+      # This section must be *after* the one configured by `libinput`
+      # for the `ScrollMethod` configuration to not be overriden
+      inputClassSections = lib.mkAfter [
+        ''
+          Identifier      "MX Ergo scroll button configuration"
+          MatchProduct    "MX Ergo"
+          MatchIsPointer  "on"
+          Option          "ScrollMethod"    "button"
+          Option          "ScrollButton"    "9"
+        ''
+      ];
+    };
+  };
+}