ipn/ipnlocal: error when trying to use exit node on unsupported platform (#13726)

Adds logic to `checkExitNodePrefsLocked` to return an error when
attempting to use exit nodes on a platform where this is not supported.
This mirrors logic that was added to error out when trying to use `ssh`
on an unsupported platform, and has very similar semantics.

Fixes https://github.com/tailscale/tailscale/issues/13724

Signed-off-by: Mario Minardi <mario@tailscale.com>

cmd/tailscale/depaware.txt

@@ -92,6 +92,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/disco                                          from tailscale.com/derp
         tailscale.com/drive                                          from tailscale.com/client/tailscale+
         tailscale.com/envknob                                        from tailscale.com/client/tailscale+
+        tailscale.com/envknob/featureknob                            from tailscale.com/client/web
         tailscale.com/health                                         from tailscale.com/net/tlsdial+
         tailscale.com/health/healthmsg                               from tailscale.com/cmd/tailscale/cli
         tailscale.com/hostinfo                                       from tailscale.com/client/web+