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/misc/introspect.js

@@ -154,7 +154,7 @@ var IntrospectService = class {
                     'is-hidden': GLib.Variant.new('b', window.is_hidden()),
                     'has-focus': GLib.Variant.new('b', (window == focusWindow)),
                     'width': GLib.Variant.new('u', frameRect.width),
-                    'height': GLib.Variant.new('u', frameRect.height)
+                    'height': GLib.Variant.new('u', frameRect.height),
                 };
 
                 // These properties may not be available for all windows: