input: Write a python function to manage a shopping cart with add and remove item functionalities.
  assert shopping_cart.add_item('apple', 3) is None
  assert shopping_cart.items == {'apple': 3}
  assert shopping_cart.add_item('banana', 2) is None
  assert shopping_cart.items == {'apple': 3, 'banana': 2}
  assert shopping_cart.remove_item('apple', 1) is None
  assert shopping_cart.items == {'apple': 2, 'banana': 2}
  assert shopping_cart.remove_item('banana', 2) is None
  assert shopping_cart.items == {'apple': 2}
output: class ShoppingCart:
      def __init__(self):
          self.items = {}
  
      def add_item(self, item, quantity):
          if item in self.items:
              self.items[item] += quantity
          else:
              self.items[item] = quantity
  
      def remove_item(self, item, quantity):
          if item in self.items:
              self.items[item] -= quantity
              if self.items[item] <= 0:
                  del self.items[item]
  
  shopping_cart = ShoppingCart()