Allow specifying fonts as CSS variables

[dstaley]

(gonna PR this soon, but wanted to make an issue anyway)

It would be nice to have the ability to specify font as a CSS variable. When font is a CSS variable (beginning with the string var(), the Google Fonts fontImports string should not be added to the <style> tag, and the font value should be not be enclosed in quotes.

{ font: 'var(--font-family-body)' }

text { font-family: ${font.startsWith('var(') ? font : `'${font}'`}, system-ui, sans-serif; }