Fixed regression on Windows systems

[MartenBE]

I did all my development on Linux. Now I was experimenting on a Windows system and reveal-md failed on me. Apparently I introduced 2 bugs for Windows:

• The new globSync functions have gotten built in support for Windows paths, so I had to explicitly configure these to use POSIX paths.
• The code I used to simulate __dirname was not crossplatform, so I looked up a crossplatform solution and found https://nodejs.org/api/url.html#url_url_fileurltopath_url (inspired by https://stackoverflow.com/questions/46745014/alternative-for-dirname-in-node-js-when-using-es6-modules )

Sorry for the bug, I only realized it now ...

[webpro]

No worries, I'm on macOS (posix) myself too. I never took the time to install a proper cross-OS test suite.