[FR] Implement File::isBundle on iOS

We need to determine if a directory is a bundle on iOS. The current implementation of juce_mac_Files.mm states:
return false; // xxx can't find a sensible way to do this without trying to open the bundle..

However the implementation is relatively straight forward:

    if(!file.isDirectory())
    {
        return false;
    }

    auto url = [NSURL fileURLWithPath:juceStringToNS(file.getFullPathName())
                           isDirectory:file.isDirectory()];

    NSNumber *result = nil;
    NSError *err = nil;
    [url getResourceValue:&result forKey:NSURLIsPackageKey error:&err];

    if(!err)
    {
        return result.boolValue;
    }

    return false;