chore: migrate side-by-side tests to html runner

[OEvgeny]

Fixes #

Changelog Entry

(no changelog entry for tests change)

Description

This slightly improves tests infrastructure by inferring title from the html file and also migrates over side-by-side tests suite.

Specific Changes

• migrated side-by-side tests to esm.sh
• updated html2 transform to support title
• migrated side-by-side tests to html2
• folded dark and light variants into single html file

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• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)