feat(SEOAudit): add new SEOAudit plugin

[alex-ketch]

This PR adds support for the SEOAudit plugin.

Example usage and configuration options can be found in the README

Preflight({
  plugins: [
    SEOAudit({
      // The base URL of the site
      baseUrl: 'https://example.org',
      // Override default rule configuration
      rules: {
        // For full list see: ./validations.tsx
        // Disable a rule
        flash: 'off',
        // Override a rule
        is_www: {
          label: 'Compact domain',
          description: 'The domain should not include www',
          severity: 'error',
          expected: false,
        },
      },
    }),
  ],
})