fix wrong name in extension route

This is in line with cwebber's proposal at
https://github.com/swicg/general/issues/22

app/controllers/api/v1/extensions_controller.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+require 'mastodon/extension'
+
+class Api::V1::ExtensionsController < Api::BaseController
+  def index
+    render json: Mastodon::Extension.all
+  end
+end

app/serializers/activitypub/actor_serializer.rb

@@ -27,7 +27,11 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   class EndpointsSerializer < ActiveModel::Serializer
     include RoutingHelper
 
-    attributes :shared_inbox
+    attributes :extensions, :shared_inbox
+
+    def extensions
+      api_v1_extensions_url
+    end
 
     def shared_inbox
       inbox_url

config/routes.rb

@@ -238,6 +238,8 @@ Rails.application.routes.draw do
           post :unmute
         end
       end
+
+      resources :extensions, only: :index
     end
 
     namespace :web do

lib/mastodon/extensions.rb

@@ -0,0 +1,17 @@
+class Mastodon::Extension
+  @@extensions = []
+
+  def self.register url
+    @extensions << Extension.new(url)
+  end
+
+  def self.all
+    @extensions
+  end
+
+  def initialize url
+    @url = url
+  end
+
+  attr_reader :url
+end