Refactor SBF field formatters so we have Theme hints and preprocessors

[DiegoPino]

Use case

This is something we will have to do at certain point, and the sooner we do it the easier it will be to theme/extend and improve our Media and Metadata Formatters. A lot of the heavy work is done now directly on the formatter itself, and we could benefit of moving some of that logic into Themes/Templates and Preprocessors. That way other modules can simply override our decisions and make, with the same logic/data, better looking and more powerful ways of displaying Digital Object assets.

How?

Add custom themes in each ::viewElements method for each formatter

public function viewElements(FieldItemListInterface $items) {
    $elements = parent::viewElements($items);
    foreach ($elements as &$element) {
      $element['#theme'] = 'formatter_strawberryfield_audio_formatter';
    }

    return $elements;
  }

Then make sure we implement hook_theme() and template_preprocess_HOOK() and then add a twig template to do the actual rendering.

The code right now is done, so this is an improvement of course, we are already generating a render array() but would be lovely to move some of that out of arrays into twig templates.

[DiegoPino]

see https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21theme.api.php/group/themeable/8.7.x