Exposed MentionsAPI from mentions service

This is necessary so that we can use it as the IMentionReportGenerator
dependency for the mentions-email-report packages

ghost/core/core/server/services/mentions/service.js

@@ -25,6 +25,8 @@ function getPostUrl(post) {
 }
 
 module.exports = {
+    /** @type {import('@tryghost/webmentions/lib/MentionsAPI')} */
+    api: null,
     controller: new MentionController(),
     didInit: false,
     async init() {
@@ -56,6 +58,8 @@ module.exports = {
             routingService
         });
 
+        this.api = api;
+
         this.controller.init({
             api,
             jobService: {