fix(browser): Set severity level for events captured by the global error handler (getsentry#4460)

Set severity level in events captured by the global `onerror`, for use in event processing (`beforeSend` or a global event processor).

Author: 7inspire

packages/browser/src/integrations/globalhandlers.ts

@@ -101,6 +101,8 @@ function _installGlobalOnErrorHandler(): void {
               column,
             );
 
+      event.level = Severity.Error;
+
       addMechanismAndCapture(hub, error, event, 'onerror');
     },
   );