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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Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 84a1aa1

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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)

[eltigerchino]

This was changed to ^5.6.3 in https://github.com/sveltejs/kit/pull/12822/files#r1803038073 to support top-level jsdoc type imports. We can probably return it back to ^5.3.3 but replace the import statement with inline type imports in the file

kit/packages/enhanced-img/src/preprocessor.js

Line 1 in 09296d0

/** @import { AST } from 'svelte/compiler' */

[eltigerchino]

I think the test apps were using TS 5.5.4 because it was needed when we upgraded them to use Svelte 5. Running pnpm check doesn't reveal the type errors now because I think the pnpm-lock file is using TS 5.7.3 for everything.

[benmccann]

Ah, I saw 5.5.3 some places and 5.5.4 some places. Perhaps we should standardize on 5.5.4 then