constructor(capacity) {
    this.capacity = capacity; // How many items to store in cache at max.
    this.nodesMap = {}; // The quick links to each linked list node in cache.
    this.size = 0; // The number of items that is currently stored in the cache.
    this.head = new LinkedListNode(); // The Head (first) linked list node.
    this.tail = new LinkedListNode(); // The Tail (last) linked list node.
  }
Creates a cache instance of a specific capacity.
@param {number} capacity