Increase support for Lightning CSS

[Lightning CSS](https://lightningcss.dev) is an extremely fast CSS parser, transformer, and minifier written in Rust. Compared with PostCSS, Lightning CSS has faster speed and smaller code bundles. At the same time, Lightning CSS also has built-in features such as `@import`, CSS modules, URL rebasing, etc.

Here is a [benchmark](https://github.com/parcel-bundler/lightningcss?tab=readme-ov-file#benchmarks) of **compile times** and **bundle sizes** for JavaScript-based tools, Lightning CSS and ESBuild:

Vite provides first-class support for Lightning CSS, and I believe this can be used as a reference:

- https://github.com/vitejs/vite/pull/12807

Start by reviewing tsup's current CSS handling and the linked Vite Lightning CSS pull request. The issue names no files, tests, or acceptance criteria, so the scope and definition of done need clarification before implementation can begin.

Written by the indexing model from the issue text.