fix(error): respect accept: text/html request header

[passionate-bram]

Requests made with the Fetch API cause sec-fetch-mode to be set, leading to these requests to be identified as JSON requests. The default error handler uses isJsonRequest to render errors. If a request were to set the Accept header to "text/html" the response should be HTML as well.

🔗 Linked issue

#1919

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

isJsonRequest is modified, its only usage (within nitro) is by the default event handler to determine when to serialize errors as HTML or JSON.

This change is needed because requests made with the Fetch API automatically get the sec-fetch-mode=cors header. This header is one of the conditions by which isJsonRequest function identifies JSON, but the header has no such meaning as far as the standard is concerned.

More importantly, the function does not provide any means to enforce an HTML response (for isJsonRequest to return false) when these fallback detections are used.

Resolves #1919

📝 Checklist

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

[pi0]

Thanks!