Coded FoodItem, ListItem, and ShoppingList.

I refined ListOfItems interface.  I also created the abstract class Item to use as a basis for FoodItem and ListItem.  I created ShoppingList which implements ListOfItems.  Included are all of my test classes.

I used the builder method instead of a constructor for ListItem because ListItem can be instantiated with a varying number of parameters.  The only required parameter is FoodItem.  It's like adding something to a real shopping list.  There's milk, which exists separately, but when you add it to a list you don't just add milk but you can also add quantity, unit of measure, and price associated with milk.  Also, by keeping FoodItem as a separate class then it will be easier to code a future enhancement of keeping a list of past items the user can choose from when creating a new or modifying an existing list.
this is a strange method - perhaps it is just the name - what's its' purpose?