docs: Document syntax-highlighting option replacing highlight-style

[cderv]

Quarto 1.9 (via Pandoc 3.8) introduces the syntax-highlighting option replacing highlight-style (quarto-dev/quarto-cli#13984). This updates documentation site-wide.

Changes

Site-wide rename of highlight-style to syntax-highlighting in YAML examples and prose across 13 files (tutorials, format pages, book output).

Shared include (_document-options-syntax-highlighting.md):

• Documents none and idiomatic special values
• Uses when-meta conditionals for format-specific content (PDF/LaTeX explains listings package, Typst explains native highlighting delegation)
• Adds a11y to the theme list (was missing)

Format-specific pages:

• pdf-basics.qmd and typst.qmd set doc-type metadata to trigger conditional content from the shared include
• typst.qmd now uses _document-options-begin.md (Typst-specific content for TOC, section numbering, and syntax highlighting moved into shared includes via when-meta conditionals)
• html-code.qmd fixes deprecated fig.alt attribute syntax to fig-alt
• revealjs/index.qmd adds a ### Syntax Highlighting subsection noting idiomatic is not supported

Historical files (changelog, _draft/) are intentionally unchanged.

Companion to quarto-dev/quarto-cli#13984

[cderv]

Wrong branch so I reopened #1874