[Feature]: add 103 Early Hints / automatic `Link` headers support

[XiNiHa]

Duplicates

• [x] I have searched the existing issues

Latest version

• [x] I have tested the latest version

Summary 💡

HTTP 103 Early Hints and the Link header provide ways to hint clients to start downloading resources before getting the main response. This can be used for improving FCP by hinting clients to start downloading route-level assets.

h3 also provides support for writing early hints on Node, so that could also be utilized.

While using early hints would be the most performant approach, support for the feature is currently limited to Node. Therefore, leveraging both would provide a safe fallback.

Examples 🌈

This feature has been (partially) implemented in Nuxt and SvelteKit.

Motivation 🔦

Better FCP!