Add G-code "backspace" support

Marlin/src/gcode/queue.cpp

     }
   #endif
 
-  buff[ind++] = c;
-  if (ind >= MAX_CMD_SIZE - 1)
-    sis = PS_EOL;               // Skip the rest on overflow
+  // Backspace erases previous characters
+  if (c == 0x08) {
+    if (ind) buff[--ind] = '\0';
+  }
+  else {
+    buff[ind++] = c;
+    if (ind >= MAX_CMD_SIZE - 1)
+      sis = PS_EOL;             // Skip the rest on overflow
+  }
 }
 
 /**