G-code line number for each serial port (for TFTs) (#18165)

Marlin/src/gcode/queue.h

@@ -35,7 +35,8 @@ public:
    * commands to Marlin, and lines will be checked for sequentiality.
    * M110 N<int> sets the current line number.
    */
-  static long last_N;
+
+  static long last_N[NUM_SERIAL];
 
   /**
    * GCode Command Queue
@@ -51,13 +52,21 @@ public:
 
   static char command_buffer[BUFSIZE][MAX_CMD_SIZE];
 
-  /*
+  /**
    * The port that the command was received on
    */
   #if NUM_SERIAL > 1
     static int16_t port[BUFSIZE];
   #endif
 
+  static int16_t command_port() {
+    return (0
+      #if NUM_SERIAL > 1
+        + port[index_r]
+      #endif
+    );
+  }
+
   GCodeQueue();
 
   /**