Make getLocaleLabelUsing usable in service provider

[johanmolen]

Right now we can not do the following within the panel service providers:

    public function panel(Panel $panel): Panel
    {
        return $panel
            ->plugins([
                SpatieLaravelTranslatablePlugin::make()
                    ->defaultLocales(
                        array_map(
                            static fn ($locale) => type($locale)->asString(),
                            config()->array('app.locales')
                        )
                    )
                    ->getLocaleLabelUsing(fn (string $locale, ?string $displayLocale = null): ?string => $locale),
            ])
    }

Because getLocaleLabelUsing is not returning the plugin itself, which is expected by the panel plugins() method.