[POC] feat(core): add Rspack builtin CSS plugin

[intellild]

Summary

This POC introduces the experimental pluginRspackBuiltinCss plugin to replace Rsbuild's css-loader, style-loader, and CssExtractRspackPlugin pipeline with Rspack's built-in CSS module types. It preserves loaders added by Sass, Less, PostCSS, Lightning CSS, Tailwind CSS, and custom plugins.

The plugin translates supported output.cssModules settings and exposes the remaining Rspack CSS parser and generator options through its own parameters:

import { pluginRspackBuiltinCss } from '@rsbuild/core';

export default {
  plugins: [
    pluginRspackBuiltinCss({
      parser: {
        url: false,
      },
      generator: {
        localIdentHashSalt: 'custom-salt',
      },
    }),
  ],
};

CSS ?url imports and Vue SFC CSS Modules are not supported by this POC and emit warnings where applicable.

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