### Description

This replaces the use case of `css_chunk_root_path` with a
`ProxiedAsset`, which allows the same kind of behavior but in a more
granular way: the user decides which assets are exposed under which
path, instead of it being a chunking context-wide setting.

### Testing Instructions

N/A

fix #4444
link WEB-856

See https://github.com/vercel/next.js/pull/48946