Allow to add CSS classes to registration link

Thanks for this enhancement. A couple of things:

* The pipeline is throwing phpcs warnings. Look like easy fixes.
* We'll need a test added to RegistrationLinkFormatterTest.php (in tests/src/Kernel/Plugin/Field/Formatter)

We are often using this great module in our projects and have the need to customize the links in theming. Therefore I am asking wether it would be more flexible to use a link render array like the following for twig link function usage in Theming? There is sometime a need to do this in theme to allow for example to add a fixed default class and adding for example modifier classes in the field formatter. This would enhance the use cases much more.

                $label = $this->getSetting('label') ?: $registration_type->label();
                $class[] = $this->getSetting('css_classes');
                $elements[] = [
                  '#title' => $label,
                  '#type' => 'link',
                  '#url' =>$url,
                  '#attributes' => [
                    'class' => $class,
                  ],
                ];

instead of (Line 155):

                $label = $this->getSetting('label') ?: $registration_type->label();
                $class[] = $this->getSetting('css_classes');
                $link = Link::fromTextAndUrl($label, $url)->toRenderable();
                $link['#attributes'] = ['class' => $class];
                $elements[] = [
                  '#markup' => $this->renderer->renderInIsolation($link),
                ];

There is a patch file for the above comment in #8

Thanks @joville I think that makes sense - the "toRenderable" function does exactly what you are proposing - except your way removes the need to render the link right away, so it simplifies the change (we can then remove the code related to rendering). And as you mentioned delaying the rendering allows for more theme flexibility for the link.

@john.oltman Great! Thank you very much for accepting my proposal and for the hint! I have tested the last merge and it does do the job as expected.