Add structured data testing for JSON-LD validation - #151

This PR implements comprehensive structured data testing using structured-data-testing-tool to validate JSON-LD scripts in built HTML files.

Implementation

Core validation script (test/build-structured-data-validate.mjs):

• Scans all HTML files in build/**/*.html for structured data
• Detects JSON-LD syntax errors and schema validation failures
• Provides clear success/failure feedback with structured data summaries
• Integrates with existing test patterns and CI pipeline

Fixture-based testing (test/fixtures-structured-data-should-fail.mjs):

• Tests both valid and invalid JSON-LD scenarios
• Ensures the validator correctly identifies parsing errors like the a"@context" syntax error mentioned in the issue
• Follows the repository's existing fixture test patterns

Package integration:

• Added structured-data-testing-tool as a dev dependency
• Integrated into main test chain via yarn test
• Provides standalone yarn test-structured-data command
• Automatically runs in CI pipeline without workflow changes
• Fixed Yarn PnP module resolution by using yarn node instead of direct node execution

Testing Results

The implementation successfully validates structured data:

// Valid JSON-LD passes validation
{
  "@context": "https://schema.org",
  "@type": "WebSite",
  "name": "My Website"
}

// Invalid JSON-LD (like a"@context") fails validation
{
  a"@context": "https://schema.org",  // Syntax error detected
  "@type": "WebSite"
}

The tool detects both JSON syntax errors and schema.org validation issues, providing robust structured data testing for any website built with this template.

Fixes #101.

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