Make the helper service public

[qzminski]

So it can be accessed e.g. via System::getContainer()->get(\Terminal42\PageimageBundle\PageimageHelper::class).

[aschempp]

As discussed, you should probably create an alias of the service and make it public in your bundle/module. That way you don't depend on this, and if you ever rewrite legacy code to dependency injection, this bundle won't have a left-over public service noone is using 😊