chore: fix path to app script on windows

[gtm-nayan]

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As noted on discord, in dev mode the hydration script inserted into pages is rendered as
import("./D:\\some\\path\\kit\\sites\\kit.svelte.dev\\.svelte-kit/generated/client/app.js")

which works for our current intents and purposes, hence why this is marked as a chore instead of a fix, but it just looks plain weird and might confuse users.

This PR makes it so that these paths have correct facing slashes and the /@fs/ prefix on both Linux and Windows.

[changeset-bot]

🦋 Changeset detected

Latest commit: 4d659d6

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[gtm-nayan]

oops forgot to take this out of draft

[Conduitry]

What effect does this have on end-users? I'd argue that, in general, chore: PRs shouldn't have changesets. Either it's a fix for a user-visible bug, or it's an internal refactoring that we don't need to tell users about and that we don't want Changesets to prompt us to cut a release for.

[gtm-nayan]

It was bordering between a fix and a chore for me. Both chrome and firefox find the file somehow even with the dot and the slashes so marked it as a chore instead. Should I remove the changeset then?

[Rich-Harris]

i think it's okay to err on the side of too many changesets — for stuff like this which does have user-observable changes (albeit changes they hopefully won't notice) there's no harm in being able to pin down the version where it changed in the unlikely eventuality that it causes problems for someone

also i'm about to cut a release so it doesn't matter too much