add option to disable experimental CssChunkingPlugin (#73286)
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.
