Add pause/resume actions on M600, M125

Marlin/Configuration_adv.h

  */
 //#define ACTION_ON_KILL "poweroff"
 
+/**
+ * Specify an action command to send to the host on pause and resume.
+ * Will be sent in the form '//action:ACTION_ON_PAUSE', e.g. '//action:pause'.
+ * The host must be configured to handle the action command.
+ */
+//#define ACTION_ON_PAUSE "pause"
+//#define ACTION_ON_RESUME "resume"
+
 //===========================================================================
 //====================== I2C Position Encoder Settings ======================
 //===========================================================================

Marlin/src/feature/pause.cpp

 ) {
   if (move_away_flag) return false; // already paused
 
+  #ifdef ACTION_ON_PAUSE
+    SERIAL_ECHOLNPGM("//action:" ACTION_ON_PAUSE);
+  #endif
+
   if (!DEBUGGING(DRYRUN) && unload_length != 0) {
     #if ENABLED(PREVENT_COLD_EXTRUSION)
       if (!thermalManager.allow_cold_extrude &&
     lcd_advanced_pause_show_message(ADVANCED_PAUSE_MESSAGE_STATUS);
   #endif
 
+  #ifdef ACTION_ON_RESUME
+    SERIAL_ECHOLNPGM("//action:" ACTION_ON_RESUME);
+  #endif
+
   #if ENABLED(SDSUPPORT)
     if (sd_print_paused) {
       card.startFileprint();