[enhancement:mdbook-typst-pdf] Render admonish icons in PDF output

[padamson]

Context

Admonish blocks now render as styled Typst blocks with colored left borders and bold titles in PDF output (KaiserY/mdbook-typst-pdf#11). However, the type-specific icons (note, tip, warning, etc.) are not rendered.

Why icons are hard

• Built-in mdbook-admonish icons are embedded as CSS background-image data URIs — not standalone files accessible to the typst-pdf backend
• Custom icons (like our reference type's book-open-variant.svg) are configured in book.toml under [preprocessor.admonish], which the typst-pdf backend doesn't read
• Bundling SVG copies of default icons in the crate adds maintenance burden

Current behavior

Admonish blocks render with colored left border + bold title. No icon.

Expected behavior

Each admonish type displays its icon (or a Unicode fallback) next to the title.

Possible approaches

• Bundle default admonish SVGs and emit #image() in the Typst block
• Read [preprocessor.admonish] config from book.toml for custom icon paths
• Use Unicode symbol fallbacks (e.g., ℹ️ for info, ⚠️ for warning)

Priority

Low — colored borders and bold titles provide sufficient visual differentiation for now.