Ensure all excluded files in _routes.json start with /

[Rich-Harris]

Looks like #6530 broke Cloudflare Pages deployments. I think this should fix it, if I understand correctly. cc @jrf0110 and @jahands for visibility

18:01:33.742 | Found _routes.json in output directory. Uploading.
18:01:33.761 | Validating asset output directory
18:01:34.611 | Deploying your site to Cloudflare's global network...
18:01:39.781 | Parsed 1 valid header rule.
18:01:44.711 | Success: Assets published!
18:01:45.627 | Error: Failed to publish your Function. Got error: Error 8000057: Rules must start with '/' in _routes.json

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🦋 Changeset detected

Latest commit: 0200658

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/adapter-cloudflare	Patch
@sveltejs/adapter-auto	Patch

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[Rich-Harris]

Not sure why the previous PR didn't trigger a Cloudflare Pages deploy, but this one did and it succeeded, so I'll merge this in order to cut a release