In nextjs, all non-API pages (other than sometimes _error) include the _app component. Up until now, we've leveraged this fact when deciding which webpack entrypoints should have sentry.server.config.js injected during serverside build; specifically, we inject into _app, _error, and all API handlers, but not any other non-API pages.

This works fine, but it means that if we want to be able to pick and choose which non-API pages get the config file injected, we're out of luck. Either we inject into _app (and therefore all non-API pages) or we don't (and therefore no non-API pages). In order to allow selective injection (which will be included in an upcoming PR), this inverts the logic: instead of _app being the only non-API page into which we inject, it is now one of the only pages we don't inject into. (The other is _document, which plays a similar role as _app does.) Given that _app and _document can't stand on their own (without the context of a page component inside of them), they don't need to have Sentry injected separately. Having it injected into the pages _app and _document wrap is sufficient.

(Note that this change only applies to serverside injection. Client-side, we still only inject into _app, as we can't be selective about which pages get instrumented on the front end given all of the global monkeypatching we do.)