M256 LCD brightness (#22478)

Marlin/src/gcode/gcode.h

@@ -191,6 +191,7 @@
  * M226 - Wait until a pin is in a given state: "M226 P<pin> S<state>" (Requires DIRECT_PIN_CONTROL)
  * M240 - Trigger a camera to take a photograph. (Requires PHOTO_GCODE)
  * M250 - Set LCD contrast: "M250 C<contrast>" (0-63). (Requires LCD support)
+ * M256 - Set LCD brightness: "M256 B<brightness>" (0-255). (Requires an LCD with brightness control)
  * M260 - i2c Send Data (Requires EXPERIMENTAL_I2CBUS)
  * M261 - i2c Request Data (Requires EXPERIMENTAL_I2CBUS)
  * M280 - Set servo position absolute: "M280 P<index> S<angle|µs>". (Requires servos)
@@ -820,6 +821,10 @@ private:
     static void M250();
   #endif
 
+  #if HAS_LCD_BRIGHTNESS
+    static void M256();
+  #endif
+
   #if ENABLED(EXPERIMENTAL_I2CBUS)
     static void M260();
     static void M261();