Add support for Lighning CSS

There were already requests for PostCSS support, but keats declined it as not being Rust-based, which is a good argument.

Well, let me introduce you all to Lightning CSS. It’s a CSS powerhouse, which can bundle, transform, and minify CSS and replace 30-ish PostCSS plugins. The most important feature for everyone would be auto-prefixing, I suppose. It can also generate source maps (including from already generated maps), which will make it a good addition to Zola’s CSS pipeline, perhaps even replace it, since one can extend its functionality with custom Visitors.

The best thing: It’s written in Rust and is available as a crate. I really urge you to look into this, because I think this is a better option than interfacing with or bootstrapping Node, or whatever nonsense it is that others do :stuck_out_tongue: