input: Write a function that converts a given Python class for an online store item into a dictionary format. The function should take an instance of the class as input and return a dictionary containing the item's ID, name, price, and description.
  
  assert to_dict_converter(StoreItem(1, "Laptop", 1200, "High-performance laptop"))=={'item_id': 1, 'name': 'Laptop', 'price': 1200, 'description': 'High-performance laptop'}
  assert to_dict_converter(StoreItem(2, "Smartphone", 800, "Latest model"))=={'item_id': 2, 'name': 'Smartphone', 'price': 800, 'description': 'Latest model'}
  assert to_dict_converter(StoreItem(3, "Headphones", 200, "Noise-cancelling"))=={'item_id': 3, 'name': 'Headphones', 'price': 200, 'description': 'Noise-cancelling'}
  assert to_dict_converter(StoreItem(4, "Smartwatch", 300, "Fitness tracker"))=={'item_id': 4, 'name': 'Smartwatch', 'price': 300, 'description': 'Fitness tracker'}
output: def to_dict_converter(item):
      return {
          'item_id': item.item_id,
          'name': item.name,
          'price': item.price,
          'description': item.description
      }