Return remaining nightlight time in json API

This returns the number of seconds remaining in the nightlight in the
"rem" field of the nightlight ("nl") state. If there is no nightlight
active, it returns -1.

wled00/json.cpp

@@ -318,7 +318,12 @@ void serializeState(JsonObject root)
   nl[F("fade")] = (nightlightMode > NL_MODE_SET); //deprecated
   nl[F("mode")] = nightlightMode;
   nl[F("tbri")] = nightlightTargetBri;
-  
+  if (nightlightActive) {
+      nl[F("rem")] = (nightlightDelayMs - (millis() - nightlightStartTime)) / 1000; // seconds remaining
+  } else {
+      nl[F("rem")] = -1;
+  }
+
   JsonObject udpn = root.createNestedObject("udpn");
   udpn[F("send")] = notifyDirect;
   udpn[F("recv")] = receiveNotifications;
@@ -326,7 +331,7 @@ void serializeState(JsonObject root)
   root[F("lor")] = realtimeOverride;
 
   root[F("mainseg")] = strip.getMainSegmentId();
-  
+
   JsonArray seg = root.createNestedArray("seg");
   for (byte s = 0; s < strip.getMaxSegments(); s++)
   {