Fix navigate into symlink dir

[gmist]

Problem: pressing Enter on a symlink to a directory opens it in $EDITOR instead of navigating
into it. This happens because io/fs.DirEntry.IsDir() returns false for symlinks because type is
fs.ModeSymlink, not fs.ModeDir. As a result checks like if e.IsDir() { return m.enterDir() } let symlink-to-dir
entries fall through into the file-open path.

Fix: added IsSymlinkToDir(e fs.DirEntry) bool on panel.Model for checks whether the target symlink to directory. For archives and non-symlinks the method returns false immediately so no extra syscall is incurred.

[kooler]

Thanks, nice fix!