fix(core): fatal events should set session as crashed (#15072)

In the Electron SDK, we have to manually set sessions as crashed when
native crashes are encountered:

https://github.com/getsentry/sentry-electron/blob/dabd6daf0c19c2a6f224d1e721eb5d58c3cf61ac/src/main/integrations/sentry-minidump/index.ts#L175

This has the downside that if you return `null` in `beforeSend`, the
event still impacts the release health crash rate.

I found that this was required because `_updateSessionFromEvent` in
`@sentry/core` does not consider the Electron native events as a crash
or an error because there are no exceptions in the event.

This PR ensures that any event with a `fatal` level is considered a
crash.

Author: timfish

packages/core/src/client.ts

@@ -758,7 +758,7 @@ export abstract class Client<O extends ClientOptions = ClientOptions> {
 
   /** Updates existing session based on the provided event */
   protected _updateSessionFromEvent(session: Session, event: Event): void {
-    let crashed = false;
+    let crashed = event.level === 'fatal';
     let errored = false;
     const exceptions = event.exception?.values;