DEV: Improve support for extending flags. (#8619)

- Ensure that the 'notify_moderators' flag is always the last flag when using custom flags.
- Support passign a custom FlagSettings object when replacing flags to reuse existing ones.

lib/plugin/instance.rb

@@ -127,8 +127,7 @@ class Plugin::Instance
   end
 
   # Applies to all sites in a multisite environment. Ignores plugin.enabled?
-  def replace_flags
-    settings = ::FlagSettings.new
+  def replace_flags(settings: ::FlagSettings.new)
     yield settings
 
     reloadable_patch do |plugin|