ref(types): Add undefined as possible event type (getsentry#6584)

Setting an `Event`'s property `type: undefined`, like we do in the SDK for `ErrorEvent`s causes a TS error, whenever the recommended `exactOptionalPropertyTypes` TS option is enabled. This option was introduced in TS 4.4. Since we're on TS 3.8, we couldn't catch this error. 

This patch changes to `EventType` type declaration to also accept `undefined` as a value, thereby resolving the TS error.

packages/types/src/event.ts

@@ -58,8 +58,12 @@ export interface Event {
   };
 }
 
-/** JSDoc */
-export type EventType = 'transaction' | 'profile';
+/**
+ * The type of an `Event`.
+ * Note that `ErrorEvent`s do not have a type (hence its undefined),
+ * while all other events are required to have one.
+ */
+export type EventType = 'transaction' | 'profile' | undefined;
 
 export interface ErrorEvent extends Event {
   type: undefined;