feat: native support for Websockets

[LukeHagar]

This PR is a replacement to #12961 with a completely different Websocket implementation using crossws that should be fully compatible with all major runtimes.

Functionality has been validated locally using basic tests in the options-2 test app.

Here is the new usage experience.
+server.js

import { error, accept } from '@sveltejs/kit';

export const socket = {
	upgrade(req) {
		 // Accept the websocket connection with a return
		return accept();

                // Reject the websocket connection with an error
                error(401, 'unauthorized');
	},

	open(peer) {
		//... handle socket open
	},

	message(peer, message) {
		//... handle socket message
	},

	close(peer, event) {
		//... handle socket close
	},

	error(peer, error) {
		//... handle socket error
	}
};

The newest implementation allows different sets of handlers to be implemented on a per-route basis. I have tested some basic uses of websockets locally to much success.

This PR is intended to:
Resolve #12358
Resolve #1491

Steps left

• Ensure handle runs before upgrading requests
• Gather feedback
• Add or update tests
• Fix the types
• Update the adapters
• Update the documentation
• Add a changeset
• Update language tools +server exports validation
• Automatic typing for sockets

---

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[changeset-bot]

🦋 Changeset detected

Latest commit: 10e9274

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@sveltejs/adapter-cloudflare-workers	Minor
@sveltejs/adapter-cloudflare	Minor
@sveltejs/adapter-node	Minor
@sveltejs/kit	Minor
@sveltejs/adapter-auto	Patch

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-12973-svelte.vercel.app/

this is an automated message

[pi0]

@LukeHagar LMK, if you need any help with this (you can reach me also on discord @pi0)

[nerg4l]

Please ignore this question if you think it's irrelevant or off-topic. I was wondering how someone could integrate Socket.IO with this native web socket implementation. I know that will only work with the node-adapter, and I found https://socket.io/how-to/use-with-nuxt#hook-the-socketio-server for Nuxt, but I got a bit stuck when I needed to pass the node request/response object.

[benmccann]

Please ignore this question if you think it's irrelevant or off-topic. I was wondering how someone could integrate Socket.IO with this native web socket implementation. I know that will only work with the node-adapter, and I found https://socket.io/how-to/use-with-nuxt#hook-the-socketio-server for Nuxt, but I got a bit stuck when I needed to pass the node request/response object.

I'll leave a response to you on the issue so as not to cross wires with all the PR review happening here