[theme-styleguide] Add support for object fontSizes

[kripod]

Summary

As highlighted in the docs of Theme UI, fontSizes may be specified as an object:

Arrays are not required, and plain objects will work as well.

Motivation

Currently, object syntax is not supported and the resoluting font sizes are too small, especially when demonstrating the contrast between colors:

[kripod]

Related: system-ui/theme-ui#646 (comment)

[LekoArts]

@kripod How does that look like in practice? So how does your fontSizes look like?
I don't see anyone using it this way.

[kripod]

I’m using sizes like ”sm”, ”md” and ”lg” instead of 1, 2 and 3.