Update font in cosmo.scss

[lynn]

Description

Source Sans Pro is now known as Source Sans 3. But the real reason I make this change is to include the italic: as it stands, a default-settings Quarto document has rather ugly-looking automatically-generated italic letters (slanted versions of the upright).

The bottom line in this screenshot shows the new, proper italic glyphs for a and g:

Checklist

I have (if applicable):

• filed a contributor agreement.
• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

[posit-snyk-bot]

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[cscheid]

Thanks!

We want to be a bit careful here because we use SCSS in unexpected places (in translations to Typst specifically), and we need to determine that these changes are safe.