fix: typings path separator on windows

[jcassidyav]

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• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
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What is the current behavior?

Use of '/' to split paths on windows results in error/ "undefined' being displayed.

What is the new behavior?

Uses path.sep as per Issue #5810.