Support root routes in route groups

[fyrts]

Currently, the following route definition always drops the locale prefix:

Route::prefix('prefix')->group(function () {
    Route::multilingual('/')->name('index');
});

localized_route('index') now always results in /prefix instead of /{locale}/prefix when applicable (regardless of settings).

I've modified MultilingualRegistrar::generatePrefixForLocale so that it doesn't drop the locale prefix when a group prefix applies.

[chinleung]

Sorry for the delay and thanks for the fix @fyrts!