Theming reworked

Theming for the pager has been completely reworked:

• \Drupal\entity_pager\EntityPagerLink and \Drupal\entity_pager\EntityPagerLinkInterface removed and logic inlined into \Drupal\entity_pager\EntityPager. This also means there have been adjustments to \Drupal\entity_pager\EntityPagerFactory.
• \Drupal\entity_pager\EntityPager::getLinks() no longer returns the count member. The variables that we used to build this item is now added to the template through template_preprocess_entity_pager() and the markup is constructed within entity-pager.html.twig – this allows easier modification, especially for theme developers.
• \Drupal\entity_pager\EntityPager::getCountWord() no longer exists, along with the CSS class name it was used to build. If this is still required, theme developers can reimplement the logic in their own pre-process function or twig template.
• \Drupal\entity_pager\EntityPager::getLinks() no longer returns render arrays but arrays with members title, url, and attributes. This allows modifications to markup within the twig template instead of necessarily having to use a pre-process function.