feat(typst.hs): Add alt text support for images in Typst writer

[mcanouil]

Introduce support for alt text in images within the Typst writer, enhancing accessibility.
Behaviour is by design consistent with LaTeX and HTML writers.

TO-DO: Check if the following files need to account for alt as well

• test/command/9945.md this might now get an alternative-text not expected at the time.

---

echo '![Caption "some title"](image.png){alt="Alt text describing the image"}' | pandoc -f markdown -t typst
echo '![](image.png){alt="Alt text describing the image"}' | pandoc -f markdown -t typst

---

This was identified in Quarto CLI that the alt-text was not forwarded.

Resolves quarto-dev/quarto-cli#13868

[mcanouil]

As expected, the old test now breaks because it is missing the alt-text generated similarly to LateX and HTML

2026-01-10T22:52:08.1982438Z     9236.md
2026-01-10T22:52:08.2923875Z       #1:                                                                                      FAIL (0.01s)
2026-01-10T22:52:08.2924508Z         --- test/command/9236.md
2026-01-10T22:52:08.2924933Z         +++ pandoc -t typst
2026-01-10T22:52:08.2925448Z         +   1 #figure(image("command/minimal.svg", alt: "minimal", width: 3in),
2026-01-10T22:52:08.2926050Z         -   1 #figure(image("command/minimal.svg", width: 3in),
2026-01-10T22:52:08.2926503Z         +   7 #figure(image("command/minimal.svg", alt: "minimal", height: 2in, width: 3in),
2026-01-10T22:52:08.2926997Z         -   7 #figure(image("command/minimal.svg", height: 2in, width: 3in),
2026-01-10T22:52:08.2927403Z         +  13 #figure(image("command/minimal.svg", alt: "minimal"),
2026-01-10T22:52:08.2927754Z         -  13 #figure(image("command/minimal.svg"),
2026-01-10T22:52:08.2928036Z         +  19 And inline:
2026-01-10T22:52:08.2928450Z         +  20 #box(image("command/minimal.svg", alt: "minimal", height: 2in)) and
2026-01-10T22:52:08.2929232Z         +  21 #box(image("command/minimal.svg", alt: "minimal")).
2026-01-10T22:52:08.2929625Z         -  19 And inline: #box(image("command/minimal.svg", height: 2in)) and
2026-01-10T22:52:08.2929994Z         -  20 #box(image("command/minimal.svg")).
2026-01-10T22:52:08.2930261Z         
2026-01-10T22:52:08.2930499Z         Use -p '/9236.md.#1/' to rerun this test only.

[gordonwoodhull]

I can't seem to make a suggested edit due to the content being markdown.

We discovered a couple other test cases in my version of this.

```
% pandoc -f markdown -t typst
![Caption](test.png){alt="A \"quoted\" phrase and C:\\path\\file"}
^D
#figure(image("test.png", alt: "A \"quoted\" phrase and C:\\path\\file"),
  caption: [
    Caption
  ]
)

```

```
% pandoc -f html -t typst
<img src="data:image/png;base64,iVBORw0KGgo=" alt="A small red dot">
^D
#box(image.decode("<svg xmlns=\"http://www.w3.org/2000/svg\" xmlns:xlink=\"http://www.w3.org/1999/xlink\"><image xlink:href=\"data:image/png;base64,iVBORw0KGgo=\" /></svg>", alt: "A small red dot"))

```

[mcanouil]

@gordonwoodhull Do you see something else regarding Quarto use?

[gordonwoodhull]

I’ve implemented a character-for-character equivalent workaround in Quarto as part of quarto-dev/quarto-cli#13857, since we probably won’t update our Pandoc again until 1.10.