fix(cssnano-preset): disable CSS counter minification

[YDKK]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

In sites generated with the default classic template, the CSS variable that counts line numbers is minified by cssnano during deployment. As a result, the feature that continues line numbering from a specific value using showLineNumbers=X does not work correctly.
This PR fixes the issue by adding a configuration to the cssnano-preset that prevents CSS counters from being minified.

Test Plan

I added a sample of this feature to the dogfooding page.

Since the /website deployment uses Rspack instead of Webpack, the following steps are required to reproduce the issue:

1. Comment out all feature->experimental_faster lines in /website/docusaurus.config.ts.
2. Run yarn build --locale en.
   • Note: The issue cannot be reproduced with yarn start because CSS minification does not occur in that mode.
3. Run npx serve website/build/.
4. Visit http://localhost:3000/tests/pages/code-block-tests#code-blocks-with-line-numbering-tests and check the line numbers.

Screenshots

Before	After

Test links

Deploy preview: https://deploy-preview-11487--docusaurus-2.netlify.app/

Related issues/PRs

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[slorber]

Thanks

I can confirm this is a bug and can reproduced with DOCUSAURUS_SLOWER=true yarn build:website:fast && yarn serve:website

This is because only the CSS counter ident is renamed, but the inline style ident is not: