Fix root path issue in Windows

"C:" is not a root directory and a CWD is used instead so append "\" to avoid this behavior.
Close #69

autoload/fern/scheme/file/fri.vim

@@ -5,14 +5,21 @@ function! fern#scheme#file#fri#to_fri(path) abort
   let path = s:Path.abspath(a:path)
   let path = s:Path.remove_last_separator(path)
   let path = s:Path.to_slash(path)
-  return printf('file:///%s', join(split(path, '/'), '/'))
+  let path = printf('file:///%s', join(split(path, '/'), '/'))
+  if s:is_windows && a:path =~# '^\w:/$'
+    let path .= '/'
+  endif
+  return path
 endfunction
 
 function! fern#scheme#file#fri#from_fri(fri) abort
   let path = s:is_windows ? a:fri.path : '/' . a:fri.path
   let path = s:Path.from_slash(path)
   let path = s:Path.abspath(path)
   let path = s:Path.remove_last_separator(path)
+  if s:is_windows && path =~# '^\w:$'
+    let path .= '\'
+  endif
   let path = empty(path) ? '/' : path
   return path
 endfunction

autoload/fern/scheme/file/provider.vim

@@ -49,6 +49,9 @@ function! s:node(path) abort
   let path = s:Path.abspath(a:path)
   let path = s:Path.remove_last_separator(path)
   let path = simplify(path)
+  if s:is_windows && path =~# '^\w:$'
+    let path .= '\'
+  endif
   let path = empty(path) ? '/' : path
   if empty(getftype(path))
     throw printf('no such file or directory exists: %s', path)