[Feature]: Native CSS Extract

[chenjiahan]

What problem does this feature solve?

Currently, Rsbuild uses different CSS implementations in Rspack mode and Webpack mode. Rspack mode uses the experimental CSS, while Webpack mode uses the mini-css-extract-plugin. Which means we need to maintain two separate sets of code.

Compared to mini-css-extract-plugin + css-loader, the capabilities of experimental CSS are weaker, such as CSS modules support, which has led to some issues. We plan to migrate to the native mini-css-extract-plugin of Rspack, polish this feature to stability as soon as possible (including the stability of its dependent new tree shaking), and unify the of CSS implementation.

What does the proposed API look like?

See: web-infra-dev/rspack#3210

Related PR: #1577