Let's add a few more useful methods to our linked list class. Wouldn't it be
useful if we could tell if our list was empty or not, as with our Stack and
Queue classes?
We should also be able to find specific elements in our linked list. Traversing
through data structures is something you'll want to get a lot of practice with!
Let's create an indexOf method that takes an element as an argument, and
returns that element's index in the linked list. If the element is not found
in the linked list, return -1.
Let's also implement a method that does the opposite: an elementAt method that
takes an index as an argument and returns the element at the given index.
If no element is found, return undefined.