feat: expose matched route

[Rich-Harris]

This is impromptu hackathon output and shouldn't be merged in its current state — it's here for mostly illustrative purposes.

For apps deployed to Vercel (and many other platforms) a SvelteKit app typically gets bundled into a single serverless function. For observability this is suboptimal since you can't group logs by route, only by path. We can fix this by exposing the matched route from SvelteKit (via a header like x-sveltekit-matched-route), and then forwarding that to the deployment platform (theoretically on Vercel via something like x-matched-route, though this isn't currently implemented on the Vercel side.

cc @tobiaslins

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Preview: https://svelte-dev-git-preview-kit-13635-svelte.vercel.app/

[Rich-Harris]

Merged #13679 which satisfies the motivating use case; we can revisit this in future if necessary