Mostly bug fixes and enhancements 🐛

Changes

• Adds support for Node.js 16.
• data-background-image now accepts multiple images (#2940).
• New Markdown config option animateLists — automatically turns all lists into stepped fragments (#2956).
• Reduce the tab size in code blocks from 8 to 2.
• More accurate calculation of which slide to jump to when clicking on the progress bar (#2836).
• Optimize DOM interactions and reduce forced layouts when exporting to PDF (#2843).

Fixes

• Video/audio inside of a fragment now stop playing when the fragment is hidden.
• Markdown is now split into individual slides by the default separator (---) as advertised.
• The r-fit-text layout helper now sizes text correctly in PDF exports.
• Fixes an issue where some slide-specific transitions were incorrectly overridden by the global transition setting.
• The has-dark-background helper class now works when using named colors for data-background-color (#2933).