modules: hardware: add mx-ergo

This adds the ability to scroll using the ball, which is way better.

modules/hardware/default.nix

@@ -5,6 +5,7 @@
   imports = [
     ./bluetooth.nix
     ./ergodox.nix
+    ./mx-ergo.nix
     ./networking.nix
     ./sound.nix
     ./upower.nix

modules/hardware/mx-ergo.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"
+        ''
+      ];
+    };
+  };
+}