[fix] change Vite's output directory from _app to client

[benmccann]

This fixes an issue that @userquin ran into trying to make vite-plugin-pwa work with SvelteKit. He was using Vite/Rollup's emit to output a service worker manifest and expecting it to be served from the root directory, but it was ending up under _app. _app is arguable only for stuff generated by SvelteKit though. I could imagine lots of other things like a robots.txt or something you might want to generate with a Vite plugin that need to go in a certain location and are impossible to put there today.

I also think the code ends up being a little nicer this was as we don't end up having to specify the app dir in quite as many places

I'm splitting this out from #5601 as that PR was becoming too unwieldy

[changeset-bot]

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

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[Rich-Harris]

This introduces a very minor bug — workers are no longer written to _app/assets/worker-xyz123.js, but are instead written to assets/worker-xyz123.js. Neither behaviour is correct (ideally it would be _app/immutable/worker-xyz123.js; that will be possible with the next release of Vite thanks to vitejs/vite#9212), but putting workers outside _app feels slightly naughtier.

Not enough to prevent merging this PR, but worth noting nonetheless