### What?
Port the webpack css chunking implementation for app dir to Turbopack.
It also computes ordered entries in module batches
Pages router also uses the app dir chunking, but that's incorrect and leads to some breaking test cases.
In app router `experimental.cssChunking` is enabled by default in
"loose" mode. This is to solve issues with CSS ordering & reduce the
number of requests for CSS resources (more information can be found in
the [original PR](https://github.com/vercel/next.js/pull/63157) and the
[docs](https://nextjs.org/docs/app/api-reference/next-config-js/cssChunking)).
However, it can lead to shared CSS from unrelated pages being included
in the initially loaded CSS.
This isn't always desireable, as expressed in
https://github.com/vercel/next.js/discussions/70168. It also leads to a
drift in behavior between dev/start, as the plugin is only implemented
in start.
This provides a third option to the plugin to disable it completely for
those who are ok with the trade-offs of not having it enabled.
Janka Uryga