fix: consistent path separator on windows

[scottmckendry]

Resolves #70

When debugging this error, I noticed that paths are being indexed with both / and \ separators. This was causing the nil value error in virtual_name.lua because it expects either one or the other.

To resolve this, I added an extra step to ensure all path / chars are replaced with \ before trying to determine the virtual path. This is only executed on Windows.

[Parsifa1]

@danielfalk Is it possible to merge this commit as soon as possible? Many thanks :)

[WayenVan]

a more solid solution is to use plenary.nvim for path operation.

[danielfalk]

I'm merging this into 0.3.x, if anybody wants to contribute a more solid solution, I'm open, but I'll take this for now.