This strips the internal routing headers added by the routing server. Based on some of the changes being introduced in https://github.com/vercel/next.js/pull/54813 this strips the headers, but this also adds some internal flags to turn this off during testing to validate correct routing beheviour.
The previous RegExp for data routes when i18n was enabled yielded a pattern like:
```
^\/_next\/data\/development\/(?<nextLocale>.+?)\/about.json$
^\/_next\/data\/development\/(?<nextLocale>.+?)\/blog/about.json$
```
But the capture group for the `nextLocale` did so greedily, where the following:
```
/_next/data/development/en-US/blog/about.json
```
Would actually match both routes.
This changes it to prevent the locale from including a `/` via `[^/]`, resulting in the new expressions:
```
^\/_next\/data\/development\/(?<nextLocale>[^/]+?)\/about.json$
^\/_next\/data\/development\/(?<nextLocale>[^/]+?)\/blog/about.json$
```