Add G-code "backspace" support

Marlin/src/gcode/queue.cpp

@@ -387,10 +387,16 @@ inline void process_stream_char(const char c, uint8_t &sis, char (&buff)[MAX_CMD
     }
   #endif
 
+  // 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
   }
+}
 
 /**
  * Handle a line being completed. For an empty line