feat: Add Sentry eventID to the response object (getsentry#1890)

Sets the eventId returned from captureException on ExpressJS middleware. This allows us to use the eventId to inform the user of a tracking ID they can use when in contact with Helpdesk. 

Implements behaviour as shown in https://docs.sentry.io/platforms/node/express/
Likely an oversight that was missed when moving from Raven to the new clients?

Author: Tommatheussen

packages/node/src/handlers.ts

@@ -271,7 +271,8 @@ export function errorHandler(): (
       next(error);
       return;
     }
-    captureException(error);
+    const eventId = captureException(error);
+    _res.sentry = eventId;
     next(error);
   };
 }