cmd/tailscale/cli: add a debug command to print the statedir

Example:

```console
$ tailscale debug statedir
/tmp/ts/node1
```

Updates #18019

Change-Id: I7c93c94179bd7b56d0fa8fe57a9129df05c2c1df
Signed-off-by: Alex Chan <alexc@tailscale.com>

ipn/localapi/debug.go

@@ -234,6 +234,13 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 		}
 	case "rotate-disco-key":
 		err = h.b.DebugRotateDiscoKey()
+	case "statedir":
+		root := h.b.TailscaleVarRoot()
+		w.Header().Set("Content-Type", "application/json")
+		err = json.NewEncoder(w).Encode(root)
+		if err == nil {
+			return
+		}
 	case "":
 		err = fmt.Errorf("missing parameter 'action'")
 	default: