fix(nuxt): mark config head as non-reactive

[danielroe]

🔗 Linked issue

resolvesnuxt/nuxt#13397, resolvesnuxt/nuxt#13847

❓ 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

In general using reactive objects outside of a Vue context is dangerous. This PR marks the nuxt configuration head inserted by Nuxt as non-reactive, which avoids a memory leak, as well as tidying up a bit of leftover bridge compatibility.

There seems ample space here for future improvement as well, including:

• identifying if there's something in vueuse/head implementation (or in our practice of reffing up the head values)
• detecting if useHead is being called outside of a vue context (that is, in a plugin) and marking entries as non-reactive if they aren't already refs/reactive.

📝 Checklist

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

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