perf(nitro): respond with early hints in node-based environments

[danielroe]

🔗 Linked issue

unjs/nitro#439

❓ 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 PR adds support for rendering Early Hints in node-based servers. We could also consider locating this elsewhere (e.g. in nitropack: unjs/nitro#439), but the particular hints we want to render are very tied to the specifics of the Nuxt renderer, and not easily portable. Thoughts/suggestions welcome.

I have opened a PR to refactor L104 -> h3 (unjs/h3#184), where it may also be used in userland (e.g. nitro middleware or plugins). If merged, we can refactor this PR to use the same utility.

If you want to test this, build the app and run:

curl -D - http://localhost:3000

It's only currently supported in Chrome 103+, and you'll need to be on a newer protocol than HTTP/1.1 to see it in action.

We currently render the headers per-request, which takes less than 1ms (tests ranged from 0.3ms -> 0.9ms). I've opened a PR in vue-bundle-renderer in case we want to statically render them to string when the renderer is created, as a further optimisation (nuxt-contrib/vue-bundle-renderer#40), but this may be unnecessary.

📝 Checklist

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

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