fix: serve files in _app from function, if not already handled

[Rich-Harris]

#10454 (comment). haven't actually tested this yet tested, it works

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Latest commit: a2f0465

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

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[dummdidumm]

Valid, but I think the same/sth similar needs to happen for the cloudflare-workers adapter