Convert notifications policies frontend code to Typescript (#29868)

app/serializers/rest/notification_policy_serializer.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class REST::NotificationPolicySerializer < ActiveModel::Serializer
+  # Please update `app/javascript/mastodon/api_types/notification_policies.ts` when making changes to the attributes
+
   attributes :filter_not_following,
              :filter_not_followers,
              :filter_new_accounts,