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

[alexander-akait]

Updates the README examples to use webpack's built-in CSS support (experiments.css + the css/auto module type) instead of chaining style-loader and css-loader, and covers that setup with tests.

Docs

• Every configuration example now sets type: "css/auto" on the rule and experiments: { css: true }, with style-loader/css-loader removed from the use arrays (11 examples, including the two source-map examples that also passed sourceMap to css-loader).
• The getting-started notes lead with the built-in support; a note after the first example keeps the style-loader/css-loader chain documented as the alternative for anyone on that setup.
• "Extracts CSS into separate files" gains the built-in CSS support as option 1 (with output.cssFilename/cssChunkFilename); mini-css-extract-plugin, Asset Modules, extract-loader and file-loader are kept and renumbered 2–5.
• The url(...) and source-map prose now refers to webpack handling the generated CSS rather than to css-loader specifically.

Tests

• test/helpers/getCompiler.js accepts css: true, which enables experiments.css, sets type: "css/auto", swaps output.library for output.cssFilename, and drops the test loader, since webpack consumes the loader result itself. A config.experiments passthrough was added too.
• New test/helpers/getCodeFromCssBundle.js reads the emitted CSS asset.
• test/loader.test.js: "should work with the built-in CSS support of webpack" for all four implementation/API combinations across both syntaxes. The snapshots also show webpack's CSS pipeline resolving the @import "./file.css" in the fixture.
• test/sourceMap-options.test.js: "should generate source maps with the built-in CSS support of webpack", asserting the emitted main.bundle.css.map.

Verification

• npm run test:base passes; snapshot regeneration only added entries, the existing snapshots are unchanged.
• npm run lint (eslint, cspell, tsc --noEmit, prettier) is clean.

No changeset is included, as this only touches documentation and tests without any runtime change.

🤖 Generated with Claude Code

https://claude.ai/code/session_013PtW7eezwuQP5epLFMrAky

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (7b81867) to head (ffa01b8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1336   +/-   ##
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  Coverage   96.67%   96.67%           
=======================================
  Files           2        2           
  Lines         901      901           
=======================================
  Hits          871      871           
  Misses         30       30           

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