Add support for HTTP/2 (h2c) in adapter-node

[bluepuma77]

Describe the problem

http/1 is slower, http/2 is faster. Currently SvelteKit does not support HTTP/2.0 out-of-the-box.

This topic has been raised before, even had a pull request, which was mainly denied because of

1. the risk of developers abusing integrated TLS to serve without a reverse proxy (?)
2. custom adapters can be implemented

Describe the proposed solution

Enable HTTP/2.0 (h2c) without TLS in SvelteKit, so users can benefit from decreased latencies and improved page load speeds.

That way a reverse proxy can handle TLS, and we can benefit from the http/2 speed improvements on the internal network.

Alternatives considered

Devs can build their own adapter to support http/2, but that is not seamless (issue), therefore default support would make life much easier and more SvelteKit apps faster.

Importance

would make my life easier

Additional Information

"As of July 2023, 36% (after topping out at just over 50%) of the top 10 million websites support HTTP/2." Wikipedia.

[Conduitry]

Please don't open new issues for things that you know already have existing issues. Duplicate of #1451.

[bluepuma77]

@Conduitry Sorry for this, I assumed the issue was closed as the pull request was denied, next time I will double check.