docs: use the built-in CSS support of webpack in examples

[alexander-akait]

Updates the documentation examples to use the built-in CSS support of webpack (the css/auto module type together with experiments.css) instead of chaining the loader with css-loader and style-loader, in line with what less-loader does, and covers the same setup with tests.

Documentation

Every webpack.config.js example now looks like this:

module.exports = {
  module: {
    rules: [
      {
        test: /\.styl$/i,
        type: "css/auto",
        use: ["stylus-loader"],
      },
    ],
  },
  experiments: {
    css: true,
  },
};

This covers "Getting Started" (with a note that experiments.css has to be enabled) and all option examples (stylusOptions object/function, sourceMap, webpackImporter, additionalData string/sync/async, implementation), as well as the examples section (normal usage, source maps, nib, importing JSON files, Stylus resolver).

Additionally:

• Added a "Using css-loader and style-loader" section — the built-in CSS support is not mandatory, so chaining with css-loader and style-loader is still documented as the alternative.
• Added a "CSS modules" section — with type: "css/auto", *.module.styl files are treated as CSS modules.
• Updated "In production" and "Extracting style sheets": the built-in CSS support extracts style sheets out of the box, output.cssFilename/output.cssChunkFilename configure the file names, and the MiniCssExtractPlugin is only needed when chaining with css-loader.

Tests

• Added test/helpers/getCssCompiler.js, which builds with experiments.css enabled and the css/auto module type, i.e. the configuration from the documentation.
• Added test/builtin-css.test.js, which checks the emitted CSS asset for a basic file, @import, url() (the referenced image is emitted as an asset), CSS modules (exported class names and the generated CSS), source maps (the .map asset and its sources) and a broken file.
• The test runner now only picks up test/*.test.js, otherwise the new JavaScript fixture is treated as a test file. The glob is passed as the last argument in test and test:coverage, because Node stops parsing its own flags after the first positional argument (npm run test:coverage still writes coverage.lcov).

npm test (lint + 153 tests) passes.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.21%. Comparing base (111ddd3) to head (12233ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #475   +/-   ##
=======================================
  Coverage   96.21%   96.21%           
=======================================
  Files           2        2           
  Lines        1269     1269           
=======================================
  Hits         1221     1221           
  Misses         48       48           

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