Add ability to handle win32 relative paths

Issue

Right now, based on codebase, library supports only posix relative paths.

Reproduction

This is correct relative path in context of this library:

../../../src

This is incorrect relative path in context of this library:

..\\..\\..\\src

To understand how the library understands these paths, you can call getType from this file:
posix path as Relative; win32 path - as Normal

So, anything that can be passed as a result by path.relative(...) in Node.js, will not work as expected on Windows machines.

Solutions

Possible solution is to add separation in the enum of types for RelativePosix and RelativeWin, like for the Absolute.. paths. But it's hard to predict what integrations with this library are going to break, so I think it can be a big deal.

Workaround

Add replacing for backward compatibility with Windows (Windows can handle posix separator):

path.relative(....).split(path.win32.sep).join(path.posix.sep);

PS. Maybe you want to handle only relative posix paths in this library, and then I think it would be cool to add information about this scenario or to close this issue with a preferable workaround.

Start with lib/util/path.js and its getType entry point, then reproduce the difference between ../../../src and ..\..\..\src using Node.js path.relative on Windows-style paths. Trace the existing path-type handling and related behavior before deciding whether win32 relative paths should be supported or the limitation and separator workaround should be documented.