feat: add fontStack option to built-in themes

[markdalgleish]

For now this is just adding the quick win of letting you opt out of SF Pro Rounded since it has similar metrics to the system font stack. Adding to this list in the future may require more updates to the low-level theme API.

[linear]

RNBW-2790 Expose font variables to built-in theme APIs

e.g. lightTheme({ accentColor: 'purple', font: 'Comic Sans MS' })

Note, this is blocked until RNBW-2862 is done.

This might need some discussion since different fonts have different metrics, meaning that you're also likely to want to adjust font sizes (and possibly line heights). Do we only support a set of pre-defined fonts (similar to accentColor , e.g. 'rounded' | 'system' | 'monospace') so we can ensure everything looks correct? Or if this is too constrained/unrealistic, is it possible that this isn't something we can easily support via the built-in themes and needs to be done with the low-level custom theme API where you have full control over text sizing?

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployments, click below or on the icon next to each commit.

rainbowkit-site – ./

🔍 Inspect: https://vercel.com/rainbowdotme/rainbowkit-site/4uQRCeYtrSuLXVL8sQLvhBfnXdrR
✅ Preview: https://rainbowkit-site-git-markdalgleish-rnbw-2790-2fe537-rainbowdotme.vercel.app

rainbowkit-example – ./

🔍 Inspect: https://vercel.com/rainbowdotme/rainbowkit-example/DkF8nNihvH3aXjFkV1tkKkEArzab
✅ Preview: https://rainbowkit-example-git-markdalgleish-rnbw-2-2bc71f-rainbowdotme.vercel.app