typst brand yml: handle font families as possible lists

[gordonwoodhull]

fixes #11933, allowing e.g.

brand:
  typography:
    headings: Barrio, Times New Roman

Everywhere we read brand.typography.*.family in typst brand yml, treat it as a possible list and use the Typst CSS font-family conversion to Typst array.

We should also consider expanding the brand yml schema to allow lists here, but commas are unambiguous in font names, as discussed in #11918 generic font families, so this is fairly safe.