Support slashed-zero font-variant

[JackCA]

It appears that the slashed-zero font-variant property is not currently supported. It would be nice it was :)

[kelset]

👋 there - as stated in the README and the issue template, this repo is for long-form discussions. I think you should be posting this as a PR against the main react native repo that the code details of such an implementation could be discussed further.

That's the only way it can be discussed & reviewed with the FB team.