Emit separate editor and content stylesheets from the build tools

[filipsobol]

🚀 Summary

Replaces the splitCss plugin with a reworked bundleCss plugin in ckeditor5-dev-build-tools. CSS must now be imported through explicit index-editor.css and index-content.css entry points, and the build emits three stylesheets: the combined bundle (preserving the JavaScript import order, where the editor and content styles of each feature sit next to each other), plus separate -editor and -content bundles.

The CSS ordering traversal now starts from the chunk facade module instead of every chunk module, which reduces the plugin's generateBundle() time on the full ckeditor5 bundle from ~1.2 s to ~30 ms with byte-identical output.

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📌 Related issues

• See https://github.com/ckeditor/ckeditor5-internal/issues/4268

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💡 Additional information

The three stylesheets are bundled by Lightning CSS from virtual entry points, so @imports are flattened and warnings are forwarded to Rolldown with correct source locations.

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