feat(core): warn if runtime config is not serializable

[danielroe]

🔗 Linked issue

nuxt/nuxt#21466
nuxt/nuxt#13992

❓ 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

Users may be under the impression they can set anything on runtimeConfig, e.g. Set(), Map(), functions, etc. This tries to check if it's a valid JSON object and if not - warn.

📝 Checklist

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

[pi0]

Nice!