DEV: Add reviewables:populate rake task (#30540)

Adds a new reviewables:populate rake task that works in a similar fashion to the existing *:populate rake tasks. The rake task creates pending reviewable of all core types, with possibility for plugins to extend the task to populate their own reviewable types.

app/models/reviewable_post.rb

@@ -13,6 +13,10 @@ class ReviewablePost < Reviewable
          created_or_edited_by.has_trust_level?(TrustLevel[4])
       return
     end
+    queue_for_review(post)
+  end
+
+  def self.queue_for_review(post)
     system_user = Discourse.system_user
 
     needs_review!(