Escape the JSON-LD against early script termination

[halskiszymon]

Follow-up to #1244.

JSON.stringify leaves < unescaped, so a </script> inside any siteConfig value would close the script tag early. Escaped to \u003c.

Raised by Copilot on the same plugin in react-native-enriched-html#782; this is the same one-line fix.

[github-actions]

WPT non-regression comparison

ERROR — the comparison did not produce a report; the test run itself likely failed.

_{Workflow run · this comment is updated on every push.}