What problem does this feature solve?

SFC compiler supports CSS preprocessors like Sass, but doesn't support additionalData like sass-loader.

What does the proposed API look like?

vite.config.js:

module.exports = {
  cssPreprocessOptions: {
    scss: {
       additionalData: '@import("variables.global.scss")'
    }
  }
};

Can you open this in vite repository please? https://github.com/vitejs/vite