[adapter-vercel] Use path/posix to resolve the relative path (issue#3163)

[Hydrophobefireman]

if we use the default path implementation, on windows we get malformed output since esmodules use "/" as the seperator

(related #3163 )

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Latest commit: 4b65cd3

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@sveltejs/adapter-vercel	Patch

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[Rich-Harris]

thanks — mind adding a changeset?