keyboard: Add "email" OSK variant for EMAIL purpose hint

Not too fancy at the moment, just an "us" keymap, with the emoji
key replaced by '@'. Can be improved in the future.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3162>

js/ui/keyboard.js

@@ -1413,6 +1413,8 @@ export const Keyboard = GObject.registerClass({
             keyboardModel = new KeyboardModel('number');
         } else if (purpose === Clutter.InputContentPurpose.PHONE) {
             keyboardModel = new KeyboardModel('phone');
+        } else if (purpose === Clutter.InputContentPurpose.EMAIL) {
+            keyboardModel = new KeyboardModel('email');
         } else {
             let groups = [groupName];
             if (groupName.includes('+'))