Fysetc LCD backlight timeout (#14265)

Marlin/src/feature/leds/leds.h

@@ -201,6 +201,17 @@ public:
     static void toggle();  // swap "off" with color
     static inline void update() { set_color(color); }
   #endif
+
+  #ifdef LED_BACKLIGHT_TIMEOUT
+    private:
+      static millis_t led_off_time;
+    public:
+      static inline void reset_timeout(const millis_t &ms) {
+        led_off_time = ms + LED_BACKLIGHT_TIMEOUT;
+        if (!lights_on) set_default();
+      }
+      static void update_timeout(const bool power_on);
+  #endif
 };
 
 extern LEDLights leds;