feat: support font-‍variant

I saw that font-‍variant support is planned in the README—I just thought I’d create an issue for myself and others to follow for when it’s released.

I really appreciate you open-sourcing the library and I enjoyed reading about the Harfbuzz and Shout-outs sections in the README. I look forward to trying it out later today!

I appreciate that! I'll get around to making issues for each of the other planned properties soon.