Improve Feathers accessibility

## What problem does this solve or what need does it fill?

Feathers is currently hardcoded to font size 14. This is too low for my eyes. But it looks like the only way to up this is by hacking in some observer that changes the font settings of every widget spawned.

## What solution would you like?

Feathers should have some blessed way of programmatically setting the font size. Ideally, also allow customizing the font itself and the background colors for even better accessibility.

## What alternative(s) have you considered?

- Leave Bevy only for the able-bodied
- Leave this to third-party apps
- Punt until we have a dedicated preferences API
- Document the observer trick in an example and bless that as the canonical way

## Additional context

Ideally, I would like editors to have an easy way of implementing something like letting the user press `CTRL +` to increase the font size and `CTRL -` to decrease it again.