[SEO Audits] Structured data is valid

**Audit group:** [Content Best Practices](#3117)
**Description**: JSON-LD structured data syntax is valid
**Failure description**: JSON-LD structured data syntax is invalid
**Help text:** Structured data contains rich metadata about a web page. The data may be used to provide more context to users in search results or social sharing. This audit is only checking for JSON-LD structured data syntax errors. For a checkup of Google search eligibility, try the [Structured Data Testing Tool](https://search.google.com/structured-data/testing-tool/u/0/).

**Success conditions:**
- the JSON can be parsed
- the keys are not duplicated
- `@` keywords are whitelisted in the JSON-LD spec
- `@context` URLs are valid
- validation using [jsonld.js](https://github.com/digitalbazaar/jsonld.js/blob/master/lib/expand.js)
- `@types` are whitelisted on schema.org
- object properties are whitelisted on schema.org

**Notes:**
- defers Google search eligibility and other more advanced validation logic to the Structured Data Testing Tool (SDTT)
- implementation logic is built into Lighthouse and not open sourced separately