sitemap.ts not working with custom pageExtensions

### Verify canary release - [X] I verified that the issue exists in the latest Next.js canary release ### Provide environment information ```bash Operating System: Platform: linux Arch: x64 Version: #1 SMP PREEMPT_DYNAMIC Sun Aug 6 20:05:33 UTC 2023 Binaries: Node: 16.17.0 npm: 8.15.0 Yarn: 1.22.19 pnpm: 7.1.0 Relevant Packages: next: 13.4.19 eslint-config-next: N/A react: 18.2.0 react-dom: 18.2.0 typescript: 5.2.2 Next.js Config: output: N/A ``` ### Which area(s) of Next.js are affected? (leave empty if unsure) App Router ### Link to the code that reproduces this issue or a replay of the bug https://codesandbox.io/p/sandbox/nervous-gould-p2j5xj ### To Reproduce Start app in dev mode `yarn dev` Go to path `/sitemap.xml` will return 404 not found if you rename the `sitemap.ts` to `sitemap.next.ts` (according to pageExtensions) it will fail as it does not know how to parse it  ### Describe the Bug When starting the app, and `pageExtensions` are set in `next.config`, the `sitemap.ts` is not working unless you add its extension. So, in next.config.js ``` pageExtensions: ['next.tsx', 'next.ts', 'next.jsx', 'next.js', 'ts'], ``` and `sitemap.ts` will work ``` pageExtensions: ['next.tsx', 'next.ts', 'next.jsx', 'next.js'], ``` and `sitemap.ts` will fail ``` pageExtensions: ['next.tsx', 'next.ts', 'next.jsx', 'next.js'], ``` and `sitemap.next.ts` will fail I guess that as `sitemap` file is a file threated by next framework, it could be handled the same way as layout, page... etc Maybe this is not a bug, it is a feature request. Apologize in that case and please move the issue to its respective category. ### Expected Behavior When you have `pageExtensions` enabled, those rules should apply to `sitemap` as well as other nextJS reserved files ### Which browser are you using? (if relevant)…