feat: set noindex header for build asset dir /_nuxt

[harlan-zw]

🔗 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)
• ✨ 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

Some users may find some of their assets within the asset build directory /_nuxt (or the directory itself) or getting indexed by Google.

For example just search google for inurl:/_nuxt

In almost all instances this is undesirable the only exception may be images that you'd like indexed but these should be included within your /public directory anyway.

This won't fix the indexing if users are using certain web servers which will automatically create directories for these files which is the case for some.

It's important to note that we DO NOT put this in robots.txt as it will cause issues with your pages being crawled and the module will now warn you if you do this.