fix: dont create data dir if it's a broken symlink

Checking if it's non-empty and not a directory gets us quite far, but
not all the way. While a working symlink would trigger the earlier
checks, a broken symlink does not.

This commit fixes the special case where ~/.local/share/nvim  already
exists but is a broken symlink. Thus, it fixes the following error on
startup:

E739: Cannot create directory /home/samuel/.local/share/nvim: file
already exists

(cherry picked from commit a6051e68a3)

runtime/plugin/rplugin.vim

@@ -14,7 +14,9 @@ function! s:GetManifestPath() abort
   let dest = stdpath('data')
   if !empty(dest)
     if !isdirectory(dest)
-      call mkdir(dest, 'p', 0700)
+      if getftype(dest) != "link"
+        call mkdir(dest, 'p', 0700)
+      endif
     endif
     let manifest_base = dest
   endif