[client,signal,management] Add browser client support (#4415)

client/iface/netstack/env_js.go

@@ -0,0 +1,12 @@
+package netstack
+
+const EnvUseNetstackMode = "NB_USE_NETSTACK_MODE"
+
+// IsEnabled always returns true for js since it's the only mode available
+func IsEnabled() bool {
+	return true
+}
+
+func ListenAddr() string {
+	return ""
+}