refactor: Add lightningcss mode for `turbopack-css` (#58471)
### What?
We are experimenting with `lightningcss`. This is about replacing
`swc_css` with `lightningcss` in turbopack, and the main reason for this
is to reduce the maintenance burden.
But when I tried, it introduced several regressions, so I'm putting it
behind an experimental flag.
You can enable `lightningcss` mode for **turbopack** by adding a flag to
the next config file.
```js
/**
* @type {import('next').NextConfig}
*/
const nextConfig = {
experimental: {
useLightningcss: true,
},
}
module.exports = nextConfig
```
Note that this is only for turbopack because we were not using `swc_css`
for non-turbopack mode of next.js
x-ref:
https://vercel.slack.com/archives/C03EWR7LGEN/p1700025496732229?thread_ts=1700019629.866549&cid=C03EWR7LGEN
### Why?
We should avoid regressions.
### How?
---
turbopack PR: https://github.com/vercel/turbo/pull/6456
Closes PACK-1966
---------
Co-authored-by: Leah <github.leah@hrmny.sh>
