nixos: hardware: rename 'trackball'

Since I do intend on configuring every trackball I own to use this
scheme, not just the MX Ergo.

modules/nixos/hardware/trackball/default.nix

@@ -0,0 +1,27 @@
+# Hold down the `next page` button to scroll using the ball
+{ config, lib, ... }:
+let
+  cfg = config.my.hardware.trackball;
+in
+{
+  options.my.hardware.trackball = with lib; {
+    enable = mkEnableOption "trackball 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 [
+        # MX Ergo
+        ''
+          Identifier      "MX Ergo scroll button configuration"
+          MatchProduct    "MX Ergo"
+          MatchIsPointer  "on"
+          Option          "ScrollMethod"    "button"
+          Option          "ScrollButton"    "9"
+        ''
+      ];
+    };
+  };
+}