fix(nextjs): Remove index signaure in captureUnderscoreErrorException argument type (#5463)

This removes the index signature in the `ContextOrProps` type in our nextjs `_error` helper function, `captureUnderscoreErrorException`, as it's causing build errors for people writing their `_error` page in TS.

Note: The problem I anticipated[1], which led me to add it in the first place, hasn't materialized in my testing. 

Fixes #5448.

[1] #5448 (comment)

Author: lobsterkatie

packages/nextjs/src/utils/_error.ts

@@ -4,10 +4,10 @@ import { addExceptionMechanism, addRequestDataToEvent } from '@sentry/utils';
 import { NextPageContext } from 'next';
 
 type ContextOrProps = {
-  [key: string]: unknown;
   req?: NextPageContext['req'];
   res?: NextPageContext['res'];
   err?: NextPageContext['err'] | string;
+  pathname?: string;
   statusCode?: number;
 };