Adapt `events` export for ES modules

Move an `events` export from being a property of the default export to a
separate named export, which is more appropriate for ES modules.

src/sidebar/services/analytics.js

 const VIA_REFERRER = /^https:\/\/(qa-)?via.hypothes.is\//;
 
-const events = {
+export const events = {
   ANNOTATION_CREATED: 'annotationCreated',
   ANNOTATION_DELETED: 'annotationDeleted',
   ANNOTATION_FLAGGED: 'annotationFlagged',
@@ -117,5 +117,3 @@ export default function analytics($window, settings) {
     events,
   };
 }
-
-analytics.events = events;