fix: use path/posix to resolve relative paths

[hyunbinseo]

Fixes #3211

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@sveltejs/adapter-cloudflare-workers	Patch

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[Conduitry]

path/posix wasn't added until Node 15.3.0. We want to support back to Node 14.13, so this should use the import { posix } from 'path' syntax that was used in #3200.