Assembler: Move the style variation specific font families from the main theme.json to the variation json file.

[matiasbenedetto]

What?

Move the style variation specific font families from the main theme.json to the variation JSON file.

Why?

Currently, the font families must be defined in the main theme.json file to load as expected in the editor when the style variation is switched. If this issue WordPress/gutenberg#59965 is fixed (in progress WordPress/gutenberg#65019), we could avoid this 'hack' that's producing the printing of not needed CSS in the frontend. So, this change would be a tiny performance improvement, and it could lead to simpler-to-define typography variations.

Related

The same issue in TT5: WordPress/twentytwentyfive#233