[TIMOB-26578] Windows: Missing trailing separator for directory in Ti.Filesystem.File#nativePath

There are a number of parity issues with the implementation of Ti.Filesystem.File#getDirectoryListing() on Windows, which can be seen in the tests now skipped there: https://github.com/appcelerator/titanium-mobile-mocha-suite/blob/master/Resources/ti.filesystem.file.test.js#L726-L746 Namely that: - calling it for a file should return null - calling for a directory that does not exist should return null (returns empty array right now) The other test ("can access resource directory files") may be failing due to Windows not appending a trailing separator while other platforms do? I tried with unix and windows separators and that didn't seem to fix it, so I don't think it's a separator mismatch issue.