[WIP] Refactor work from PR #117 to reduce package imports

[Copilot]

Recreating PR #117: Underline Hover Animations

Based on Discussion #117, implementing CSS custom properties for animated underline hover effects.

Original Author Credit: Temani Afif (@css_afif) - Original CodePen Demo

Plan Checklist

• Create src/props.highlights.js with underline hover animation CSS custom properties
• Add the new highlights module to the build pipeline (build/props.js)
• Add the new highlights module to the main index (src/index.js)
• Update the CSS index file (src/index.css)
• Add package.json exports for the new module
• Update tests to include the new properties count
• Run build and verify output
• Run tests to ensure nothing is broken
• Run code review and security checks

Key Design Goals (Minimal Implementation)

• No new dependencies (pure CSS custom properties)
• Follow existing patterns in the codebase
• Use shared base variables for customization (--highlight-color, --highlight-size)
• Provide in/out states for smooth hover transitions

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