Add Typescript types for some API objects (#26602)

app/serializers/rest/relationship_serializer.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class REST::RelationshipSerializer < ActiveModel::Serializer
+  # Please update `app/javascript/mastodon/api_types/relationships.ts` when making changes to the attributes
+
   attributes :id, :following, :showing_reblogs, :notifying, :languages, :followed_by,
              :blocking, :blocked_by, :muting, :muting_notifications,
              :requested, :requested_by, :domain_blocking, :endorsed, :note