Do not show the Global Styles sidebar in edit site if theme doesn't support theme.json

[oandregal]

Steps to reproduce the bug:

• Activate a FSE theme that doesn't have a theme.json
• Go to the edit site.

The expected behavior is that the "Global Styles" sidebar shouldn't be shown, as the theme hasn't declared support.

[youknowriad]

theme.json is not just about global styles. What happens if I use a theme.json to enable/disable some features in the editor but maybe I still don't want the global styles sidebar?

[oandregal]

Depending on where we land at #27295 (comment) the exact checks we need for this may vary. Probably it's worth investigating this issue after 27295 is tackled.

[skorasaurus]

Is this still an issue ?

[ramonjd]

I think this issue is still relevant. See discussion on #52750