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 a test app.

Here is the new usage experience.
+server.js

export const socket = {
	upgrade(req) {
		//... handle socket upgrade
		return {
			headers: {}
		};
	},

	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: 7791759

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[Rich-Harris]

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

this is an automated message

[eltigerchino]

Is returning void or a ResponseInit important to cancel the websocket connection? This would be a breaking change.

[LukeHagar]

Its as I mentioned on Discord, returning a Response aborts the Websocket upgrade, and returning a headers object (or maybe more?) accepts the upgrade request, I'm not certain on the void bit here but that was their typing.

[LukeHagar]

I do think there is a path to improving the dev experience here, we have an issue logged on the crossws repo to discuss further.

[eltigerchino]

Related to unjs/crossws#88

[LukeHagar]

Same, would love to keep this as is
unjs/crossws#90

[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.