DEV: create new_post_moved event trigger when moving posts (#24005)

This change adds a new event trigger (new_post_moved) when the first post in a topic is moved to a new topic.

Plugins that listen for the new_post_moved event now have an easy way to update old data based on the post id.

app/models/post_mover.rb

@@ -277,6 +277,7 @@ class PostMover
     new_post.custom_fields = post.custom_fields
     new_post.save_custom_fields
 
+    DiscourseEvent.trigger(:first_post_moved, new_post, post)
     DiscourseEvent.trigger(:post_moved, new_post, original_topic.id)
 
     # we don't want to keep the old topic's OP bookmarked when we are