Support font-display in @font-face rule

[jpt]

I have started seeing font-display: swap in @font-face at-rules more often in performance-minded web apps. This addresses a Flash of Invisible Text. Optional support would be great - I put the issue here instead of core because it would be especially nice as a flag for the CLI. If it's something you'd want to implement, let me know if I can help. Cheers!

[RoelN]

It would be good to add a Sane Default™ so devs will either use that, or remember to set it to what they prefer.

Great issue, thanks @jpt!