`panel.menu.site.current` option should automatically ignore other paths

Currently, you have to supply a ‘current’ function to every custom menu entry. This should ideally be done automatically. Especially when plugins register ‘pages/xxx’ areas as menu entries.

Currently the easiest solution would be to use a helper class like this:
https://github.com/tobimori/kirby-spielzeug/blob/main/classes/Menu.php