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

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Latest commit: f70aea1

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[Rich-Harris]

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

this is an automated message

[eltigerchino]

Suggested change

	export const socket = {
	/** @type {import("crossws").Hooks} */
	export const socket = {

Should we re-export the Hooks type provided by crossws? So it becomes something like

/** @type {import("@sveltejs/kit").Socket} */

[LukeHagar]

I was hoping we could actually type it automatically the same way the GET functions are automatically typed when exported from the same file, I would love to handle that for the users.

I think Ben mentioned that @dummdidumm was the typing expert here, they maybe be able to provide some insight on the best way to get this the last mile here.

[eltigerchino]

I think the automatic typing is part of the Svelte language tools VS Code extension so we would still need the manual typing available here.

[pi0]

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

[LukeHagar]

@benmccann @dummdidumm @eltigerchino I think things are rapidly approaching done here, I would really appreciate if someone could lead the charge in getting any additional eyes on this needed, additionally if we could look to update the types however its needed to get the exports typed.

Thank you all so much for your help getting to this point!

I'm going to be rebasing things shortly