Add Shell.FileList(path)

So I can bind to the Unix-filesystem-layer version of ls that landed
with [1].  I've spun this layer off into it's own file to avoid
crowding shell.go.

[1]: ipfs/kubo#1348

unixfs.go

@@ -0,0 +1,49 @@
+package shell
+
+import (
+	"encoding/json"
+	"fmt"
+
+	cmds "github.com/ipfs/go-ipfs/commands"
+	cc "github.com/ipfs/go-ipfs/core/commands"
+	unixfs "github.com/ipfs/go-ipfs/core/commands/unixfs"
+)
+
+// FileList entries at the given path using the UnixFS commands
+func (s *Shell) FileList(path string) (*unixfs.LsObject, error) {
+	ropts, err := cc.Root.GetOptions([]string{"file", "ls"})
+	if err != nil {
+		return nil, err
+	}
+
+	req, err := cmds.NewRequest([]string{"file", "ls"}, nil, []string{path}, nil, unixfs.LsCmd, ropts)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := s.client.Send(req)
+	if err != nil {
+		return nil, err
+	}
+	if resp.Error() != nil {
+		return nil, resp.Error()
+	}
+
+	read, err := resp.Reader()
+	if err != nil {
+		return nil, err
+	}
+
+	dec := json.NewDecoder(read)
+	out := unixfs.LsOutput{}
+	err = dec.Decode(&out)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, object := range out.Objects {
+		return object, nil
+	}
+
+	return nil, fmt.Errorf("no object in results")
+}