Creates a functional Linked List object in Ruby! This implementation includes the following methods to go with the object:

• #append(value) -- adds a new node containing value to the end of the list
• #prepend(value) -- adds a new node containing value to the start of the list
• #size -- returns the total number of nodes in the list
• #head -- returns the first node in the list
• #tail -- returns the last node in the list
• #at(index) -- returns the node at the given index
• #pop -- removes the last element from the list
• #contains?(value) -- returns true if the passed in value is in the list and otherwise returns false.
• #find(value) -- returns the index of the node containing value, or nil if not found.
• #to_s -- represent LinkedList objects as strings, so you can print them out and preview them in the console. Format is: ( value ) -> ( value ) -> ( value ) -> nil
• #insert_at(value, index) -- inserts a new node with the provided value at the given index.
• #remove_at(index) -- removes the node at the given index.