Add Z_PROBE_LOW_POINT to prevent damage

Marlin/src/config/default/Configuration.h

@@ -777,6 +777,8 @@
 #define Z_CLEARANCE_BETWEEN_PROBES  5 // Z Clearance between probe points
 //#define Z_AFTER_PROBING           5 // Z position after probing is done
 
+#define Z_PROBE_LOW_POINT          -2 // Farthest distance below the trigger-point to go before stopping
+
 // For M851 give a range for adjusting the Z probe offset
 #define Z_PROBE_OFFSET_RANGE_MIN -20
 #define Z_PROBE_OFFSET_RANGE_MAX 20