Expose the time of the last change to the LED state (#17222)

quantum/led.h

@@ -61,6 +61,9 @@ void led_set_kb(uint8_t usb_led);
 bool led_update_user(led_t led_state);
 bool led_update_kb(led_t led_state);
 
+uint32_t last_led_activity_time(void);    // Timestamp of the LED activity
+uint32_t last_led_activity_elapsed(void); // Number of milliseconds since the last LED activity
+
 #ifdef __cplusplus
 }
 #endif