feat(fonts): set default fallbacks

[florian-lefebvre]

Changes

• After discussing with @delucis, we came to the conclusion that having sans-serif as a default fallback makes sense so users can benefit from automatic fallback generation by default

Testing

N/A

Docs

Will be documented properly alongside all defaults

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[ematipico]

Please make sure to update the RFC with this information. I just checked it and it doesn't mention sans-serif being fallback.

[florian-lefebvre]

Ah yes I forgot, I'll add it now