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/environment.js

@@ -269,7 +269,7 @@ function init() {
             let { stack } = new Error();
             log(`Usage of object.actor is deprecated for ${klass}\n${stack}`);
             return this;
-        }
+        },
     });
 
     St.set_slow_down_factor = function (factor) {