Replace the f32 font_size value of TextStyle with a FontSize enum

[ickshonpe]

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

[caniko]

Hello, I feel that this is a very important feature! What is missing is agreement on how to handle font atlases considering we will automatically generate new fonts when the parent is resized.