✨ Fan tachometer support (#23086)

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

Marlin/src/gcode/gcode.h

@@ -156,6 +156,7 @@
  * M121 - Disable endstops detection.
  *
  * M122 - Debug stepper (Requires at least one _DRIVER_TYPE defined as TMC2130/2160/5130/5160/2208/2209/2660 or L6470)
+ * M123 - Report fan tachometers. (Requires En_FAN_TACHO_PIN) Optionally set auto-report interval. (Requires AUTO_REPORT_FANS)
  * M125 - Save current position and move to filament change position. (Requires PARK_HEAD_ON_PAUSE)
  *
  * M126 - Solenoid Air Valve Open. (Requires BARICUDA)
@@ -736,6 +737,10 @@ private:
   static void M120();
   static void M121();
 
+  #if HAS_FANCHECK
+    static void M123();
+  #endif
+
   #if ENABLED(PARK_HEAD_ON_PAUSE)
     static void M125();
   #endif