Updates to theme.json

[oandregal]

Superseedes #175

This PR updates the theme.json to the latest format, according to the two PRs that landed in Gutenberg recently:

• Update theme.json: split global block selector into root and defaults gutenberg#28533
• Update theme.json shape: make settings & style top-level keys gutenberg#28110

How was the format before those PR landed:

{
  "global": {
    "settings": { ... },
    "styles": { ... }
  },
  "core/paragraph": {
    "settings": { ... },
    "styles": { ... }
  }
}

How it's now:

{
  "settings": {
    "defaults": { ... },
    "root": { ... },
    "core/paragraph": { ... }
  },
  "styles": {
    "root": { ... },
    "core/paragraph": { ... }
  }
}

There's more details and info in the docs.