`panel.menu`: default for `current`

When defining custom menu entries, adding the current callback can be a bit tedious. We could simplify this by adding a default that at least covers basic entries with just a link prop.

               'current' => function (string $current) use ($props): bool {
                    $path = App::instance()
                        ->request()
                        ->path()
                        ->toString();
                    return Str::contains($path, $props['link']);
                }