chore: posixify paths used on the server during development

[teemingc]

splitting out more changes from #15574

This PR ensures these paths use the correct slash separators which is important in the Cloudflare Vite environment

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from c91b319:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/c91b319af90cf5debdb17d05dba46196e1fb17b2

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16294

[changeset-bot]

⚠️ No Changeset found

Latest commit: c91b319

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16294-svelte.vercel.app/