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/Collection/TalkCollection.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_talks\Collection;
+
+use Drupal\opdavies_talks\Entity\Node\Talk;
+use Drupal\paragraphs\ParagraphInterface;
+use Tightenco\Collect\Support\Collection;
+
+final class TalkCollection extends Collection {
+
+  /**
+   * Return the events for the talks in the Collection.
+   *
+   * @return Collection|ParagraphInterface[]
+   */
+  public function getEvents(): Collection {
+    return $this->flatMap(fn(Talk $talk): Collection => $talk->getEvents());
+  }
+
+}