Font Family: make it work like any other block support with presets

[oandregal]

The block supports mechanism works by attaching classes (when the property has preset values such as colors or font-sizes) or inline styles (when it doesn't such as line-height or when the user selects custom values for colors or font-sizes).

The link color was an exception to this model because it addresses the inner markup of a block. We experimented with using CSS Custom Properties for that one but that approach had some issues and we're now exploring an alternative at #31488

The font family also uses CSS Custom Properties but there's no reason it shouldn't work like the rest of the block supports with presets. We should make it work like the rest in preparation for stabilizing it #31416 (comment)

[youknowriad]

I think this one is still experimental right? So we can remove it from the 5.8 must haves?

[oandregal]

Yes, forgot about the board (had already triaged it at #20331).

[ockham]

(Some WIP here: #31910)