Add an emergency-command parser to MarlinSerial (supporting M108)

Add an emergency-command parser to MarlinSerial's RX interrupt.

The parser tries to find and execute M108,M112,M410 before the commands disappear in the RX-buffer.

To avoid false positives for M117, comments and commands followed by filenames (M23, M28, M30, M32, M33) are filtered.

This enables Marlin to receive and react on the Emergency command at all times - regardless of whether the buffers are full or not. It remains to convince hosts to send the commands. To inform the hosts about the new feature a new entry in the M115-report was made. "`EMERGENCY_CODES:M112,M108,M410;`".

The parser is fast. It only ever needs two switch decisions and one assignment of the new state for every character.

One problem remains. If the host has sent an incomplete line before sending an emergency command the emergency command could be omitted when the parser is in `state_IGNORE`.
In that case the host should send "\ncommand\n"

Also introduces M108 to break the waiting for the heaters in M109, M190 and M303.

Rename `cancel_heatup` to `wait_for_heatup` to better see the purpose.

Marlin/Marlin_main.cpp

@@ -160,7 +160,7 @@
  * M105 - Read current temp
  * M106 - Fan on
  * M107 - Fan off
- * M108 - Cancel heatup and wait for the hotend and bed, this G-code is asynchronously handled in the get_serial_commands() parser
+ * M108 - Stop the waiting for heaters in M109, M190, M303. Does not affect the target temperature.
  * M109 - Sxxx Wait for extruder current temp to reach target temp. Waits only when heating
  *        Rxxx Wait for extruder current temp to reach target temp. Waits when heating and cooling
  *        IF AUTOTEMP is enabled, S<mintemp> B<maxtemp> F<factor>. Exit autotemp by any M109 without F
@@ -1105,9 +1105,12 @@ inline void get_serial_commands() {
         }
       }
 
-      // If command was e-stop process now
-      if (strcmp(command, "M112") == 0) kill(PSTR(MSG_KILLED));
-      if (strcmp(command, "M108") == 0) wait_for_heatup = false;
+      #if DISABLED(EMERGENCY_PARSER)
+        // If command was e-stop process now
+        if (strcmp(command, "M108") == 0) wait_for_heatup = false;
+        if (strcmp(command, "M112") == 0) kill(PSTR(MSG_KILLED));
+        if (strcmp(command, "M410") == 0) stepper.quick_stop();
+      #endif
 
       #if defined(NO_TIMEOUTS) && NO_TIMEOUTS > 0
         last_command_time = ms;
@@ -4533,10 +4536,14 @@ inline void gcode_M105() {
 
 #endif // FAN_COUNT > 0
 
-/**
- * M108: Cancel heatup and wait for the hotend and bed, this G-code is asynchronously handled in the get_serial_commands() parser
- */
-inline void gcode_M108() { wait_for_heatup = false; }
+#if DISABLED(EMERGENCY_PARSER)
+
+  /**
+   * M108: Stop the waiting for heaters in M109, M190, M303. Does not affect the target temperature.
+   */
+  inline void gcode_M108() { wait_for_heatup = false; }
+
+#endif
 
 /**
  * M109: Sxxx Wait for extruder(s) to reach temperature. Waits only when heating.
@@ -4811,7 +4818,9 @@ inline void gcode_M111() {
 /**
  * M112: Emergency Stop
  */
-inline void gcode_M112() { kill(PSTR(MSG_KILLED)); }
+#if DISABLED(EMERGENCY_PARSER)
+  inline void gcode_M112() { kill(PSTR(MSG_KILLED)); }
+#endif
 
 #if ENABLED(HOST_KEEPALIVE_FEATURE)
 
@@ -5991,13 +6000,15 @@ inline void gcode_M400() { stepper.synchronize(); }
  * This will stop the carriages mid-move, so most likely they
  * will be out of sync with the stepper position after this.
  */
-inline void gcode_M410() {
-  stepper.quick_stop();
-  #if DISABLED(DELTA) && DISABLED(SCARA)
-    set_current_position_from_planner();
-  #endif
-}
 
+#if DISABLED(EMERGENCY_PARSER)
+  inline void gcode_M410() {
+    stepper.quick_stop();
+    #if DISABLED(DELTA) && DISABLED(SCARA)
+      set_current_position_from_planner();
+    #endif
+  }
+#endif
 
 #if ENABLED(MESH_BED_LEVELING)
 
@@ -6953,9 +6964,11 @@ void process_next_command() {
         gcode_M111();
         break;
 
-      case 112: // M112: Emergency Stop
-        gcode_M112();
-        break;
+      #if DISABLED(EMERGENCY_PARSER)
+        case 112: // M112: Emergency Stop
+          gcode_M112();
+          break;
+      #endif
 
       #if ENABLED(HOST_KEEPALIVE_FEATURE)
 
@@ -6974,9 +6987,11 @@ void process_next_command() {
         KEEPALIVE_STATE(NOT_BUSY);
         return; // "ok" already printed
 
-      case 108:
-        gcode_M108();
-        break;
+      #if DISABLED(EMERGENCY_PARSER)
+        case 108:
+          gcode_M108();
+          break;
+      #endif
 
       case 109: // M109: Wait for temperature
         gcode_M109();
@@ -7261,9 +7276,11 @@ void process_next_command() {
           break;
       #endif // ENABLED(FILAMENT_WIDTH_SENSOR)
 
-      case 410: // M410 quickstop - Abort all the planned moves.
-        gcode_M410();
-        break;
+      #if DISABLED(EMERGENCY_PARSER)
+        case 410: // M410 quickstop - Abort all the planned moves.
+          gcode_M410();
+          break;
+      #endif
 
       #if ENABLED(MESH_BED_LEVELING)
         case 420: // M420 Enable/Disable Mesh Bed Leveling