Add support for CSS client references (#5397)
### Description
This PR changes CSS assets to support the CSS client references use case
of Next.js, where .css assets in the RSC graph should be replaced with a
client reference assets.
CSS assets are now processed in two steps:
1. Processing the CSS source asset into either a Global or a Module CSS
asset.
2. Inside of Global/Module assets, processing the CSS source asset into
a CSS asset.
By default, (2.) will create a leaf CSS asset. But in Next.js RSC, this
step will be replaced with creating a CSS client reference instead. This
client reference can then be detected when walking the RSC graph, and a
client CSS chunk can be produced from there.
### Testing Instructions
N/A
