Support NumFDs on Windows

[braydonk]

Describe the solution you'd like
I would like to add support for the NumFDs method in Windows.

Describe alternatives you've considered
OpenFiles is implemented for Windows, however doesn't cover all the information; Windows process handles can be more than just

Additional context
I opened #1454 as a draft to demonstrate a solution that I think makes sense for getting the full number of handles. However this may not be the way to go, because there is some nuance between "Handle" in Windows and "File Descriptor". In POSIX we're only considered about fds, but in Windows we may like to know about all handles.

FWIW, psutil simply has different methods for Windows and Linux (num_handles and num_fds respectively). Do we want to try and make a distinction like that here? I don't know how this library likes to handle differences in interface between platforms.

[braydonk]

This was done in #1712 so I'll close this.