Add support for pause and resume action commands on M600

Marlin/Configuration_adv.h

@@ -1441,6 +1441,14 @@
  */
 //#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/Marlin_main.cpp

@@ -6497,6 +6497,10 @@ inline void gcode_M17() {
   ) {
     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 &&
@@ -6729,6 +6733,10 @@ inline void gcode_M17() {
       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();