A theme.json schema check in CI

A typo in a theme.json key is silently ignored, which means a setting that does nothing looks exactly like a setting that works.

- name: Validate theme.json
  run: |
    npx ajv-cli validate 
      -s https://schemas.wp.org/wp/6.1/theme.json 
      -d theme.json --strict=false
    for f in styles/*.json; do
      npx ajv-cli validate -s ... -d "$f" --strict=false
    done

The published schema is versioned by WordPress release, so pinning it to the minimum supported version catches a key that only exists in a later one. It is the cheapest possible check for a file that is otherwise verified by loading a page and looking at it, and it catches the class of error — a misspelled key — that a visual check cannot.