fix(types): Use Sentry event type instead of dom one

#4527 introduced
Envelope types. In that patch, we included the `Event` DOM type, which
is not declared in all TS environments accidentally - we meant to
include Sentry's `Event` type.

This patch explicitly introduces Sentry's Event type by importing it in,
fixing the build errors that folks were having.

Author: AbhiPrasad

packages/types/src/envelope.ts

@@ -1,3 +1,4 @@
+import { Event } from './event';
 import { SentryRequestType } from './request';
 import { SdkInfo } from './sdkinfo';
 import { Session, SessionAggregates } from './session';