Replace the f32 font_size value of TextStyle with a FontSize enum

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

Replacing the `font_size` field of `TextStyle` with an enum similar to Bevy UI's `Val` type would enable users to set responsive font sizes that depend on the size of the viewport or the text's parent container.

## What solution would you like?

#9524

Read the referenced issue #9524 first, then locate TextStyle and the existing font_size uses. The issue names no files or tests; done means replacing the f32 value with a FontSize enum that supports sizes responsive to the viewport or parent container.

Written by the indexing model from the issue text.