Add generic font families

[Unisay]

As per https://developer.mozilla.org/en/docs/Web/CSS/font-family

• serif
• sans-serif
• monospace
• cursive
• fantasy
• system-ui

[CLAassistant]

All committers have signed the CLA.

[JordanMartinez]

I think this is fine to add. We already have sansSerif and this isn't a core library. If we're going to support these, perhaps we should support all the ones mentioned in the link above?

[thomashoneyman]

Yes, I think we should support the full set of generic font families as listed in the MDN docs.

[thomashoneyman]

I’m a little surprised that it’s not a sum type, since they’re an enumeration, but perhaps it’s to avoid breaking changes if more are added in the future? I would be happy changing to a sum type also.

[JordanMartinez]

The downside to adding a sum type is that it would be a breaking change every time a new font family is added. Since this is likely just being used to ensure the string value (e.g. "monospace") is spelled correctly, I'm not sure we want to do that.

[thomashoneyman]

I suppose I don’t know how often these change (CSS properties tend to be stable). Typically for things that don’t change often and which are enumerations I’d reach for a sum type but I’m also happy with a newtype here. I still think we should add the remaining cases, though.

[JordanMartinez]

Superceded by #136