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.

[emfax]

I've been deep in to this lately. I intended to submit a PR to support variable fonts, but Bevy's take on fonts is this,

/// A font *face* (e.g. Fira Sans Semibold Italic) is part of a font *family* (e.g. Fira Sans),
/// and is distinguished from other font faces in the same family
/// by its style (e.g. italic), its weight (e.g. bold) and its stretch (e.g. condensed).

Also, cosmic_text uses swash for glyph rasterisation, which does support variable fonts, but I don't think cosmic_text is able to.

So I don't think it's a simple fix.

I've been working on an alternative text plugin as a proof of concept. I decided that the best way to support variable fonts is with SDF font rendering, which requires a bit more doing. I'm aiming to have it done by the end of this week.