fix: run reroute in a middleware function before split functions

[teemingc]

fixes #11879

See #12296 for context

This PR adds a middleware that runs reroute beforehand so that, when there are multiple functions, the correct one is invoked instead of not matching any function matchers.

The Vercel middleware uses a simplified version of the rewrite helper from @vercel/functions to avoid pulling in the whole bundle.

• rewrite docs - https://vercel.com/docs/functions/edge-middleware/middleware-api#rewrites
• rewrite implementation - https://github.com/vercel/vercel/blob/cefda60a603d60cc35e4697c36e751cca411e6bb/packages/functions/src/middleware.ts#L101
• edge middleware example - https://github.com/vercel/examples/tree/main/build-output-api/edge-middleware

The Netlify middleware resolves the URL then passes it to the next handler.

There's currently an issue with Vercel rewrites discarding the SvelteKit form action in the URL because the query parameter doesn't have a value vercel/vercel#12902

TODOS

• implement Vercel middleware
• add split test app on Vercel

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from df404ce:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/df404ce1540794d3c0267050e096080eb3257175

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16590

[changeset-bot]

🦋 Changeset detected

Latest commit: df404ce

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

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-netlify	Major
@sveltejs/adapter-vercel	Major
@sveltejs/kit	Minor

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16590-svelte.vercel.app/

[teemingc]

using a major here to make it more noticeable that folks will have this extra middleware function deployed

[Rich-Harris]

is this right, or does it also apply to apps that are implicitly split because of differing configs?

[teemingc]

Yeah, the latter; as long as there's more than one function

[Rich-Harris]

I'm not 100% sure about this API. It's called getReroutePath but it returns a path to the universal hooks.js file which also includes transport (and whatever else we add in future) so at the very least it should be named getUniversalHooksPath or something.

But we probably only want to expose the reroute logic, which leaves me wondering if it makes sense to do something like this instead:

import { applyReroute } from 'WHATEVER';

export default function middleware(request) {
	return applyReroute(request);
}

[teemingc]

Mm would it be a good idea to have a builder.generateReroute() which bundles up the reroute logic and returns a string instead of writing to disk? and it returns undefined if the adapter doesn't need to care about reroute?