Add `@font-feature-values`

[yisibl]

We already have the font-variant-alternates property, but the @font-feature-values associated with it is missing. In CSS, the two need to work together.

• Safari 16.2 https://webkit.org/blog/13591/webkit-features-in-safari-16-2/#:~:text=Interop%202022%20score.-,font%20features,-Safari%2016.2%20adds
• Chrome 111 https://chromestatus.com/feature/5070560260784128
• Firefox 34 https://bugzilla.mozilla.org/show_bug.cgi?id=975744
• Demo: https://developer.mozilla.org/en-US/docs/Web/CSS/font-variant-alternates#examples

[Schweinepriester]

+1, at least related to #3920