On windows, os.relativePath returns path as is when roots are different

[demotomohiro]

Before making this PR, os.relativePath returns wrong path when a root of path and a root of base are different.

import os

echo relativePath(r"c:\foo", r"d:\bar")
echo relativePath(r"\\foo\bar\x", r"\\foo\baz\")

output:

..\..\c:\foo
..\bar\x

This PR uses PathIsSameRootW or PathIsSameRootA windows API to compare roots of input paths.
If they were not same root, returns path as is.
But PathIsSameRootW or PathIsSameRootA seems don't support DOS device paths.

This code test PathIsSameRootW.
https://gist.github.com/demotomohiro/81702d8a0bf951d6a46b9954093d69bb

APIs in pathcch.h header supports DOS device paths but there is not API to compare root of paths and they are only available on windows 8 or newer.

Implementing PathIsSameRoot that supports DOS device paths can be larger code than this PR.

[demotomohiro]

I changed code so that os.relativePath compare roots of input paths without using windows API.
Because windows API is not available at compile time or nimscript and os.relativePath can return wrong relative at compile time.