fix(middleware): improve handling of custom Next.js `basePath` (#5109)
* fix(middleware): improve handling of custom nextjs basePath
* fix(middleware): improve extraction of nextjs base path from req.nextUrl
* adapt to req.nextUrl.basePath
* Fix indent
* Add middleware test for custom-base and simplified code a little bit
* Fix indent
* Add another test
* Rename basePath and nextJsBasePath
* Fix lint error