Typst writer: add image alt text support

[gordonwoodhull]

Simple fix implemented by Claude.

Since there is no corresponding issue, here are the details:

• Add alt parameter to Typst image() calls for accessibility (PDF/UA compliance)
• Follows same pattern as LaTeX writer: checks for explicit alt attribute first, falls back to image caption
• If neither is available, omits alt

Background

Typst supports PDF/UA-1 accessibility standards via the alt parameter on image() calls. Without this, accessible PDFs generated through Typst fail validation with "missing alt text" errors.

Before:

#figure(image("cat.png"),
  caption: [
    A cat sleeping
  ]
)

After:

#figure(image("cat.png", alt: "A cat sleeping"),
  caption: [
    A cat sleeping
  ]
)

The implementation mirrors the existing LaTeX writer behavior where:

1. Explicit {alt="..."} attribute takes priority
2. Falls back to stringified image caption
3. Omits alt parameter when no alt text is available

[gordonwoodhull]

Oops, this is almost the same as #11394 - the only difference in output is that @mcanouil puts alt first after the filename, which I agree with. I'll close this and suggest a couple more tests there.