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/perf/hwtest.js

@@ -114,7 +114,7 @@ function *run() {
     Scripting.scriptEvent('desktopShown');
 
     let interfaceSettings = new Gio.Settings({
-        schema_id: 'org.gnome.desktop.interface'
+        schema_id: 'org.gnome.desktop.interface',
     });
     interfaceSettings.set_boolean('enable-animations', false);