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

[alexander-akait]

Updates the README examples to webpack's built-in CSS support and adds test coverage for that setup.

Documentation

Every config example now uses experiments.css and the css/auto module type instead of chaining style-loader and css-loader:

{
  test: /\.less$/i,
  type: "css/auto",
  use: ["less-loader"],
}
// + experiments: { css: true }

The surrounding prose was updated to match, not just the code blocks:

• Getting Started notes that experiments.css has to be enabled, and links to the loader-chain section for anyone who still needs it.
• Normal usage explains css/auto vs css vs css/module.
• New Using css-loader and style-loader section keeps the old chain documented.
• In production / Extracting style sheets now explain that built-in CSS extracts style sheets on its own (pointing at output.cssFilename), and mention MiniCssExtractPlugin only for the loader-chain path. The dead extract-loader link was dropped.
• CSS modules gotcha became a proper CSS modules section with a working example; the url(...) caveat is kept.

Tests

New test/built-in-css.test.js (7 tests), driven by a new test/helpers/getCssCompiler.js that builds a compiler with experiments.css, type: "css/auto" and output.pathinfo: false (so snapshots aren't polluted by webpack's path banner). Coverage:

• plain CSS output and @import resolution (relative and through node_modules)
• url() asset emission and rewriting
• CSS modules class-name renaming
• lessOptions and additionalData passthrough
• source maps: the generated map references the original .less file and carries its source content
• error reporting on a broken file

Fixtures live in test/fixtures/built-in-css/. pathinfo was added to .cspell.json.

Notes

• The existing tests keep using testLoader.cjs to capture raw loader output. Converting them would mean asserting against webpack's CSS-parser output rather than less-loader's, which is a weaker assertion, so built-in CSS coverage was added alongside instead of replacing them.
• The CSS modules test uses a .module.less entry rather than a JS entry importing it: Node's test runner treats every .js file under test/ as a test file, so a JS fixture importing .less would fail as a "test". Narrowing the runner's glob would break test:coverage, since Node ignores flags placed after positional patterns.
• No changeset — docs and tests only, no source change.

Full test suite and npm run lint (eslint, cspell, tsc, prettier) pass locally.

🤖 Generated with Claude Code

https://claude.ai/code/session_017wwBzDKPnXaC6vFomCtu42

---

Generated by Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: f33f4c4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.61%. Comparing base (45b1042) to head (f33f4c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #609   +/-   ##
=======================================
  Coverage   96.61%   96.61%           
=======================================
  Files           2        2           
  Lines         649      649           
=======================================
  Hits          627      627           
  Misses         22       22           

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