DEV: move discourse_dev gem to the core. (#13360)

And get avatar images from `discourse_dev_assets` gem.

lib/discourse_dev.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module DiscourseDev
+  def self.config
+    @config ||= Config.new
+  end
+
+  def self.settings_file
+    File.join(root, "config", "settings.yml")
+  end
+
+  def self.root
+    File.expand_path("..", __dir__)
+  end
+end