thinkpads: configure second monitor for t480

hosts/thinkpads/i3.nix

@@ -5,8 +5,21 @@
     brightnessctl
   ];
 
-  xsession.windowManager.i3.config.keybindings = lib.mkOptionDefault {
+  xsession.windowManager.i3.config = {
+    keybindings = lib.mkOptionDefault {
       "XF86MonBrightnessUp" = "exec --no-startup-id brightnessctl set +5%";
       "XF86MonBrightnessDown" = "exec --no-startup-id brightnessctl set 5%-";
     };
+
+    workspaceOutputAssign = [
+      {
+        output = "eDP-1";
+        workspace = "1";
+      }
+      {
+        output = "HDMI-2";
+        workspace = "2";
+      }
+    ];
+  };
 }