`[css-fonts-5]` Add generic `font-family` for symbols/emoji

We could do with a generic font family (either ui-symbol or ui-emoji) to expose fonts such as "Segoe UI Symbol" and "Apple Symbols" as set on the user's computer or browser.

https://www.w3.org/TR/css-fonts-5/#generic-font-families

[svgeesus]

Since CSS Fonts 4 just dropped the emoji generic font family in favor of the font-variant-emoji property, this does not seem likely.

• [css-fonts] Drop the emoji font-family #9359