ipn: add user pref for running web client

This is not currently exposed as a user-settable preference through
`tailscale up` or `tailscale set`.  Instead, the preference is set when
turning the web client on and off via localapi. In a subsequent commit,
the pref will be used to automatically start the web client on startup
when appropriate.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -810,6 +810,9 @@ func TestPrefFlagMapping(t *testing.T) {
 		case "Egg":
 			// Not applicable.
 			continue
+		case "RunWebClient":
+			// TODO(tailscale/corp#14335): Currently behind a feature flag.
+			continue
 		}
 		t.Errorf("unexpected new ipn.Pref field %q is not handled by up.go (see addPrefFlagMapping and checkForAccidentalSettingReverts)", prefName)
 	}