docs: deprecate style-loader in favor of webpack's built-in CSS support

[alexander-akait]

Summary

webpack injects CSS into the DOM itself — experiments.css defaults to "auto" since 5.109.0, and the exportType: "style" parser option covers the default injectType: "styleTag" — so style-loader is no longer needed. Adds a deprecation notice at the top of the README linking the Native CSS guide, its migration guide and the codemod. The notice also names the three options with no native equivalent (attributes, insert, styleTagTransform), so anyone relying on them knows to stay.

What kind of change does this PR introduce?

docs

Did you add tests for your changes?

n/a — README and .cspell.json only, no code change.

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Nothing further — https://webpack.js.org/loaders/style-loader/ is generated from this README, so the notice appears there on the next docs build. The /loaders index already marks the loader deprecated.

Use of AI

Yes — Claude Code drafted the notice and checked every linked anchor against the guides in webpack.js.org and the experiments.css version against webpack's schema and changelog. Reviewed and edited by me before pushing.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (5d8caa5) to head (ee71d79).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #653   +/-   ##
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  Coverage   94.93%   94.93%           
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  Files          15       15           
  Lines         296      296           
  Branches      149      149           
=======================================
  Hits          281      281           
  Misses         15       15           

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