cleanup: Require "dangling" commas

Since ES5, trailing commas in arrays and object literals are valid.
We generally haven't used them so far, but they are actually a good
idea, as they make additions and removals in diffs much cleaner.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805

js/ui/extensionSystem.js

@@ -236,7 +236,7 @@ var ExtensionManager = class {
             path: dir.get_path(),
             error: '',
             hasPrefs: dir.get_child('prefs.js').query_exists(null),
-            canChange: false
+            canChange: false,
         };
         this._extensions.set(uuid, extension);