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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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.

Tests

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

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 1ac7ccf

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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]

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

[eltigerchino]

What's the reason for removing the private access modifier?

[eltigerchino]

There's some logic that's missing and might be important before the handle hook runs (mostly to do with populating the event object and whatever before_handle does). They're in the respond function but not the new resolve function.

kit/packages/kit/src/runtime/server/respond.js

Lines 176 to 177 in 461aa95

	/** @type {import('types').TrailingSlash | void} */
	let trailing_slash = undefined;

kit/packages/kit/src/runtime/server/respond.js

Lines 242 to 245 in 461aa95

	// if `paths.base === '/a/b/c`, then the root route is `/a/b/c/`,
	// regardless of the `trailingSlash` route option
	if (url.pathname === base || url.pathname === base + '/') {
	trailing_slash = 'always';

kit/packages/kit/src/runtime/server/respond.js

Line 275 in 461aa95

trailing_slash = node.trailingSlash;

kit/packages/kit/src/runtime/server/respond.js

Lines 299 to 322 in 461aa95

	if (state.before_handle || state.emulator?.platform) {
	let config = {};
	/** @type {import('types').PrerenderOption} */
	let prerender = false;
	if (route.endpoint) {
	const node = await route.endpoint();
	config = node.config ?? config;
	prerender = node.prerender ?? prerender;
	} else if (route.page) {
	const nodes = await load_page_nodes(route.page, manifest);
	config = get_page_config(nodes) ?? config;
	prerender = get_option(nodes, 'prerender') ?? false;
	}
	if (state.before_handle) {
	state.before_handle(event, config, prerender);
	}
	if (state.emulator?.platform) {
	event.platform = await state.emulator.platform({ config, prerender });
	}
	}

kit/packages/kit/src/runtime/server/respond.js

Lines 330 to 345 in 461aa95

	const { cookies, new_cookies, get_cookie_header, set_internal } = get_cookies(
	request,
	url,
	trailing_slash ?? 'never'
	);
	cookies_to_add = new_cookies;
	event.cookies = cookies;
	event.fetch = create_fetch({
	event,
	options,
	manifest,
	state,
	get_cookie_header,
	set_internal
	});