Add a theme.json editor

[creativecoder]

I'm seeing that even theme builders who work mainly from the Global styles interface still need to directly edit theme.json files for some settings.

Adding a theme.json editor to Create Block Theme could have multiple benefits:

• Help avoid needing to switch to a text editor to make these changes
• Show theme.json structure and valid settings to speed up workflows and provide built in documentation

A super basic version of this would be enabling the existing theme file editor to access theme.json.

A more advanced version could use the schema of theme.json to show what options are available and valid for the area of the file you are working in, perhaps using autocomplete, or having a more structured interface, like checkboxes for booleans and drop-downs when there is a limited set of valid choices.