Support font collections

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

OpenType font files may contain multiple fonts (e.g. regular, bold, italic, thin). When using such a file for displaying text with Bevy, only one of the available fonts can be used.

## What solution would you like?

There could be a way to specify which font of the collection must be used, maybe in `Font` or `TextStyle`.

## What alternative(s) have you considered?

Currently the only alternative is splitting the file into separate files for each font.