fix: render json errors for CORS requests

[danielroe]

🔗 Linked issue

nuxt/nuxt#19093

❓ 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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

As $fetch is setting Accept: */* we can't detect JSON request purely by content-type. I think sec-fetch-mode might allow us to make another smart guess - CORS requests should probably also be JSON.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

Good idea!