Implement `size-adjust` property

[valadaptive]

The size-adjust CSS property allows you to set scaling factors for specific font families. This is useful for harmonizing the size of text written in different fonts (something similar is used in egui for adjusting emoji fonts' sizes).

This can't be implemented at a higher level by just scaling the font size for a given style, because you can specify a stack of multiple fonts as part of a style, and we only want the size adjustment to apply to a specific one.