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

[alexander-akait]

Replaces style-loader/css-loader/mini-css-extract-plugin in the documentation examples with webpack's built-in CSS support (experiments.css + the css/auto module type), and adds tests for that setup.

README

• Every webpack config example now sets experiments: { css: true } and type: "css/auto" on the rule, and drops style-loader/css-loader from the loader chain (along with the importLoaders options that only existed for css-loader).
• CSS Modules section rewritten around css/auto (*.module.css) with a css/module variant for treating all matched files as modules; the stale css-loader#modules link now points at the CSS Modules spec.
• Extract CSS section no longer uses mini-css-extract-plugin — extraction is native, configured through output.cssFilename / output.cssChunkFilename.
• The sass-based examples (sourceMap, implementation) had their test regex corrected to /\.s[ac]ss$/i — they matched .css while running sass-loader.
• Added a note that this requires webpack >= 5.87.0, and that css-loader/style-loader still work if preferred, with postcss-loader placed before them.

Tests

• New test/helpers/getCssCompiler.js compiles fixtures through experiments.css + css/auto with no css-loader-like loader in the chain.
• New test/builtInCss.test.js covers plain CSS, CSS modules, SugarSS, execute with postcss-js, source maps in the emitted main.css.map, and the invalid-syntax error path. Fixtures in test/fixtures/builtin-css/.
• test/fixtures/esparser/runManual.mjs no longer references css-loader (it is not a devDependency, so the manual runner was broken) and uses the built-in CSS pipeline.

The existing unit tests keep test/helpers/testLoader.js. With built-in CSS, webpack's CSS parser regenerates the module source as a RawSource and drops the loader's source map at the module level, so the assertions in sourceMap.test.js about the loader's own map (sources, sourcesContent, absolute paths) cannot be made against it — converting them would trade precise loader-output assertions for webpack's composed asset maps.

Full suite passes (132 passed, 4 skipped) and npm run lint is clean.

---

Generated by Claude Code

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ Unknown (b31e0a3)
• ✅ login: alexander-akait / name: alexander-akait (b31e0a3)

One or more co-authors of this pull request were not found. You must specify co-authors in commit message trailer via:

Co-authored-by: name <email>

Supported Co-authored-by: formats include:

1. Anything <id+login@users.noreply.github.com> - it will locate your GitHub user by id part.
2. Anything <login@users.noreply.github.com> - it will locate your GitHub user by login part.
3. Anything <public-email> - it will locate your GitHub user by public-email part. Note that this email must be made public on Github.
4. Anything <other-email> - it will locate your GitHub user by other-email part but only if that email was used before for any other CLA as a main commit author.
5. login <any-valid-email> - it will locate your GitHub user by login part, note that login part must be at least 3 characters long.

Alternatively, if the co-author should not be included, remove the Co-authored-by: line from the commit message.

Please update your commit message(s) by doing git commit --amend and then git push [--force] and then request re-running CLA check via commenting on this pull request:

/easycla

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.23%. Comparing base (438644b) to head (29fd6fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #740   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files           3        3           
  Lines         357      357           
  Branches      118      118           
=======================================
  Hits          315      315           
  Misses         38       38           
  Partials        4        4           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.