feat(gui/kbd): add configurable ellipsis keybinding

- Introduce `gui.kbd.ellipsis.enable` option for keybinding.
- Move related xkb configuration to a dedicated module for clarity.
- Simplify `default.nix` by delegating ellipsis setup to the new module.

modules/home/gui/kbd/default.nix

@@ -0,0 +1,25 @@
+{ config, lib, ... }:
+
+with lib;
+let
+  cfg = config.metacfg.gui.kbd;
+in
+{
+  options.metacfg.gui.kbd = {
+    ellipsis = mkEnableOption "ellipsis keybinding for period key";
+  };
+
+  config = mkIf cfg.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
+    '';
+  };
+}