ref(types): Add undefined as possible value to EventType

[Lms24]

Fixes #6575

As described in the issue, setting an Event's property type: undefined, like we do in the SDK for ErrorEvents

sentry-javascript/packages/types/src/event.ts

Lines 68 to 70 in aad38ab

	export interface ErrorEvent extends Event {
	type: undefined;
	}

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

Note:
The initially suggested fix to change the Event's type type to type: EventType | undefined causes TS errors all over the place as it would require us to set type explicitly to undefined whenever we create error events. I'd vote not to do this as it is a breaking change because it makes an optional parameter non-optional. Furthermore, it requires a lot of changes and also increases bundle size