cleanup: Always use single quotes

We have made good progress with transitioning to the new style,
to the point where we can complete it with a final push.

Start with changing the remaining places that still use double
quotes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

js/ui/popupMenu.js

@@ -757,7 +757,7 @@ export class PopupMenuBase extends Signals.EventEmitter {
         } else if (menuItem instanceof PopupBaseMenuItem) {
             this._connectItemSignals(menuItem);
         } else {
-            throw TypeError("Invalid argument to PopupMenuBase.addMenuItem()");
+            throw TypeError('Invalid argument to PopupMenuBase.addMenuItem()');
         }
 
         menuItem._setParent(this);