typst brand yaml: handle lists of fonts in typography element definitions

[gordonwoodhull]

Split out from #11929

---
brand:
  typography:
    fonts:
      - family: Barrio
        source: google
      - family: Roboto
        source: google
    base: Roboto
    headings: Barrio, Times New Roman
---

## Base font

{{< lipsum 1 >}}

## Heading is `#context text.font`{=typst}

Base is `#context text.font`{=typst}

::: {style="font-family: Barrio"}
{{< lipsum 1 >}}
:::
#show heading: set text(font: ("Barrio", "Times New Roman"))

Typst passes the first test, since caching of font resources is independent of what is used.

But it fails the second test due to not anticipating a list. 🤦

Producing

#show heading: set text(font: "Barrio, Times New Roman", )

which should be

#show heading: set text(font: ("Barrio", "Times New Roman"))

## Heading is `#context text.font`{=typst}

produces

But it's really Typst's global fallback font, Linux Libertine. 😝

Very important for multilingual text, will fix and backport.

Originally posted by @gordonwoodhull in #11929