#3840 - allow user to pass any path separator as --chunk_output…

[DerGernTod]

fixes #3840 - allow user to pass any path separator as --chunk_output_path_prefix

as a user, you might want to use unix-style path separators always, since they're also compatible with windows. doing so however causes issues when writing source maps, since maybeCreateDirsForPath only creates output directories if path separators are in os-style.

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[DerGernTod]

looks like the cla bot doesn't recognize my account, i'll try to resubmit a new pr