perf(nuxt): only inject preload helper when webpack is used

[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

This started out as a simple small perf improvement but ended up fixing a couple of bugs as well. The changes are:

• only inject the server preload plugin when webpack builder is used (it adds a needless overhead with vite)
• update NuxtOptions schema so it correctly shows resolved value of builder
• update resolver for builder so it respects a custom builder value

📝 Checklist

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

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