Support for Debug Codes - Dnnn (#19225)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/gcode/gcode.cpp

@@ -923,6 +923,10 @@ void GcodeSuite::process_parsed_command(const bool no_ok/*=false*/) {
 
     case 'T': T(parser.codenum); break;                           // Tn: Tool Change
 
+    #if ENABLED(MARLIN_DEV_MODE)
+      case 'D': D(parser.codenum); break;                         // Dn: Debug codes
+    #endif
+
     default:
       #if ENABLED(WIFI_CUSTOM_COMMAND)
         if (wifi_custom_command(parser.command_ptr)) break;