feat(input-sources): add custom xkb option for ellipsis

Introduces a new XKB configuration for typing the ellipsis symbol. Updates dconf settings and XDG config files to enable and map the custom "mod:ellipsis" option.

homes/x86_64-linux/harald@x1/default.nix

@@ -56,6 +56,18 @@
     };
   };
 
+  dconf.settings."org/gnome/desktop/input-sources".xkb-options = [ "mod:ellipsis" ];
+  xdg.configFile."xkb/symbols/mod".text = ''
+    xkb_symbols "ellipsis" {
+      key <AB09> { [ period, greater, ellipsis, division ] };
+    };
+  '';
+  xdg.configFile."xkb/rules/evdev".text = ''
+    ! option = symbols
+      mod:ellipsis    = +mod(ellipsis)
+    ! include %S/evdev
+  '';
+
   xdg.enable = true;
   xdg.mime.enable = true;
 }