Swap to a variable font in bevy_feathers

Originally posted by @cart in https://github.com/bevyengine/bevy/pull/19730#discussion_r2170113915:

> I think its worth considering [Firava](https://github.com/hellogreg/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.