fix(fs, model): improve symlink resilience in file shortcut (#10739)

Ensure file was a file before the shortcut as well as after... (This was
implied when talking to a correct implementation, but not enforced.)

Make our file opening operations safe by default by ensuring the last
path component is not a symlink.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs_windows.go

@@ -21,6 +21,8 @@ import (
 	"golang.org/x/sys/windows"
 )
 
+const alwaysOpenFlags = 0 // no extra flags
+
 var errNotSupported = errors.New("symlinks not supported")
 
 func (BasicFilesystem) ReadSymlink(path string) (string, error) {