Swap to a variable font in bevy_feathers

[alice-i-cecile]

Originally posted by @cart in #19730 (comment):

I think its worth considering Firava (a variable font variant of Fira Sans), rather than checking in each font variant. Provided it works, it would give users maximum flexibility and (ideally) cut down on a bit on complexity.
I believe we can render variable fonts, however it looks like we aren't currently wiring up "font width", so idk how functional it would be in the short term. We don't need to resolve this now. Although committing to that path now would mean we aren't adding these fonts to the history, which would be nice.

[alice-i-cecile]

Note: we first need to check if Bevy properly supports variable fonts. Then, fix that if needed. Then wire it into bevy_feathers.