Include nextConfig in edge runtime via separate manifest (#86769)
A separate manifest (`required-server-files.js`, like `_buildManifest.js`) so that we the bundler doesn't have to inline the whole config.
The Node.js runtime routes already get their nextConfig at runtime by reading `required-server-files.js`.
Closes https://github.com/vercel/next.js/pull/86630
Context for `initializeCacheHandlers`: https://github.com/vercel/next.js/pull/76130