Support multiple font families within an element

[DuncanMacWeb]

Is your feature request related to a problem? Please describe.
Approximately 2,140 characters are used in less than 2% of the content on our pages that we need to render to PDF. The remaining 98% of pages use just 200 characters. We don’t want to send the unneeded characters to render the 98% of PDFs that don’t need them.

Describe the solution you'd like
Improve the font fallback mechanism to pull characters from fallback fonts specified using the CSS font-family: foo, bar, baz syntax (or the equivalent in React Native style objects)

Describe alternatives you've considered
We have considered putting all the characters any of our content could possibly need into a single font file, but it would just be too large and a waste of network bandwidth.

Relates to #377, #402

[SachinRupani]

Any update on supporting multiple font families / font files?