heading-family should default to mainfont in typst-brand-yaml.lua

[gordonwoodhull]

fixes

• Typst brand settings always triggered for creating title #13474

Alternative PR to #13475.

A simpler approach, but one that diverges further from the Pandoc default.typst.

Process mainfont as if it sets brand.typography.base.family.

I think this is cleaner and safer, and doesn't have any questions of precedence.

But I'll sleep on it before merging this or the other one.

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