fix(nuxt): warn about legacy and invalid plugins

[danielroe]

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

📚 Description

Following #5630, plugins which take a second argument for inject will silently fail. This at least adds a warning if they are being used. Example: https://github.com/moritzsternemann/vue-plausible/blob/main/src/nuxt-plugin.ts (hotfix PR).

📝 Checklist

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

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