feat: improved Content-Signal support

[harlan-zw]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

We originally added support for the Content-Usage directive. Since then, Cloudflare has introduced Content-Signal, and they both have a different implementation so we add support for both with their own types and validation.

export default defineNuxtConfig({
  robots: {
    groups: [
      {
        userAgent: '*',
        allow: '/',
        contentSignal: {
          'ai-train': 'no',
          search: 'yes'
        }
      }
    ]
  }
})