Making push_at_n()

Now that we can insert nodes at the front (push_front()) and back (push_back()), we need to add the ability to insert a node at a specific position in the linked list.

This function allows us to place a node at the nth position, where n is a valid index. However, this function should only work if the position exists in the list. The function will return true if the node was inserted and false if something went wrong.