Add UNKNOWN_Z_NO_RAISE option

With this option enabled, Z won't ever be raised until after `G28` has been completed, and it won't raise if Z becomes unknown. This is good for machines whose beds fall when Z is powered off.

Marlin/src/gcode/calibrate/G28.cpp

@@ -221,9 +221,15 @@ void GcodeSuite::G28(const bool always_home_all) {
 
     #endif
 
-    if (home_all || homeX || homeY) {
+    #if ENABLED(UNKNOWN_Z_NO_RAISE)
+      const float z_homing_height = axis_known_position[Z_AXIS] ? Z_HOMING_HEIGHT : 0;
+    #else
+      constexpr float z_homing_height = Z_HOMING_HEIGHT;
+    #endif
+
+    if (z_homing_height && (home_all || homeX || homeY)) {
       // Raise Z before homing any other axes and z is not already high enough (never lower z)
-      destination[Z_AXIS] = Z_HOMING_HEIGHT;
+      destination[Z_AXIS] = z_homing_height;
       if (destination[Z_AXIS] > current_position[Z_AXIS]) {
 
         #if ENABLED(DEBUG_LEVELING_FEATURE)