[Feature]: builtin-lightningcss-loader add support for `visitor` option

[ckken]

What problem does this feature solve?

https://rspack.dev/zh/guide/features/builtin-lightningcss-loader#options https://lightningcss.dev/transforms.html

What does the proposed API of configuration look like?

LightningcssLoaderOptions 增加 visitor

[JSerFeng]

It's cool but not that high priority for us at present, and if using JavaScript visitor, there will be interop between Rust and JavaScript, so the performance gain of builtin-loader is gone, in this case you can try calling lightningcss-node binding yourself, or you can try this loader from community

We will support visitors in the future, but still, not high priority for us at present, of course we are happy to help if you are willing to contribute for this feature

[ckken]

Without visitor, from a business perspective, built-in functions are useless, such as px to vw, etc.

[ckken]

https://empjs.dev/plugin/tool/plugin-lightningcss.html At present, the processing method of postcss has been rewritten. I just want to switch to the built-in function, but the gap is a bit big