Implement filament change continuous purge

Marlin/Configuration_adv.h

@@ -921,6 +921,7 @@
   #define FILAMENT_UNLOAD_RETRACT_LENGTH 13   // (mm) Unload initial retract length.
   #define FILAMENT_UNLOAD_DELAY 5000          // (ms) Delay for the filament to cool after retract.
   #define FILAMENT_UNLOAD_PURGE_LENGTH 8      // (mm) An unretract is done, then this length is purged.
+  //#define ADVANCED_PAUSE_CONTINUOUS_PURGE   // Purge continuously up to the purge length until interrupted.
 
   #define PAUSE_PARK_NOZZLE_TIMEOUT 45        // (seconds) Time limit before the nozzle is turned off for safety.
   #define FILAMENT_CHANGE_ALERT_BEEPS 10      // Number of alert beeps to play when a response is needed.