[css-fonts-5] Allow tweaking word-spacing at per-fallback granularity

[litherum]

This issue is split off from #126

The use case is being able to tweak the look of fallback fonts while web fonts are loading. The goal is to reduce the effects of the visual reflow when the web font does finally load (aka "flashiness")

The goal is to allow pages the ability to say something like "for this fallback font, use word spacing X, but for this other fallback font, use word spacing Y."

In #126 (comment) we resolved to do this sort of thing via @font-face descriptors.

In #5533 we resolved to do this for letter spacing. This bug tracks a similar feature for word spacing.