fix: correctly resolve $app/state (alternative)

[dummdidumm]

Alternative to #13181, fixes #13179

Uses the same approach as the other virtual modules to differentiate between client and server

Note that I haven't tested this out with an actual Vite 5 project yet (if someone could that would be great; I probably won't have much time to do so)

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[dominikg]

do we want to keep the separate client and server files and rely on them being side effect free and the unused import treeshaken or consolidate all code into the index file and branch inside this one file?

looking at https://github.com/sveltejs/kit/blob/main/packages/kit/src/runtime/app/stores.js maybe app/state.js instead of a subdirctory with 3 files would follow that pattern more closely?

[dummdidumm]

I kept it like this because it made code easier to reason about - TBH we should maybe go the other way around if we want to consolidate. And also, if we require Vite 6 in SvelteKit 3, we could switch back to the package.json approach, as that seems to work there.

[eltigerchino]

It works 👍🏼 Vite 5 example with this PR