refactor: Add lightningcss mode for `turbopack-css` (#6456)
### Description
To make WIPs small, I'm putting ligtningcss as a mode.
x-ref:
https://vercel.slack.com/archives/C03EWR7LGEN/p1700025496732229?thread_ts=1700019629.866549&cid=C03EWR7LGEN
### Testing Instructions
Closes PACK-1965
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
