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.

lib/discourse_dev/reviewable_post.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require "discourse_dev/reviewable"
+require "faker"
+
+module DiscourseDev
+  class ReviewablePost < Reviewable
+    def populate!
+      @posts.sample(2).each { |post| ::ReviewablePost.queue_for_review(post) }
+    end
+  end
+end