Add built-in static syntax highlighting using skylighting

[srid]

Replaces client-side JavaScript syntax highlighters (highlight.js, prism.js) with static highlighting using skylighting, the same library used by Pandoc.

Key improvements:

• Code blocks are now highlighted at build time using skylighting (140+ languages supported)
• No JavaScript required, resulting in faster page loads and better compatibility
• Works seamlessly with live preview mode
• Both light and dark mode themes included
• Configurable via syntaxHighlighting option (default: true)

The documentation has been updated to reflect the new approach, and legacy JavaScript snippets have been removed from example configurations.

Resolves #337
Uses srid/heist-extra#10

[srid]

https://emanote.srid.ca/syntax-highlighting