Add TalkCollection, move logic for getting events

Add a `TalkCollection` which extends Laravel/Tighten's, and add a method
there for getting the events from the talks. This makes this logic more
reusable and also makes the code in the `TalkCounter` service simpler.

web/modules/custom/talks/src/Service/TalkCounter.php

@@ -5,7 +5,6 @@ declare(strict_types=1);
 namespace Drupal\opdavies_talks\Service;
 
 use Carbon\Carbon;
-use Drupal\opdavies_talks\Entity\Node\Talk;
 use Drupal\opdavies_talks\Repository\TalkRepository;
 use Drupal\paragraphs\ParagraphInterface;
 
@@ -22,8 +21,9 @@ final class TalkCounter {
 
     return $this->talkRepository
       ->findAllPublished()
-      ->flatMap(fn(Talk $talk) => $talk->getEvents())
-      ->filter(fn(ParagraphInterface $event) => $event->get('field_date')->getString() <= $today)
+      ->getEvents()
+      ->filter(fn(ParagraphInterface $event) => $event->get('field_date')
+        ->getString() <= $today)
       ->count();
   }