Add TypeScript goody for traversing a binary tree in level order

[miorel]

Similar to #157, but level order! Since for level order, the level of the node is often important I think a good API might be roughly:

function traverseLevelOrder<T>(root: T | null | undefined): Generator<T[], void, void>;

In other words, we can yield an array of the nodes on each level.

Why a generic function?

While investigating how linked lists work on LeetCode, I noticed that the objects we get aren't necessarily instances of the listed classes like ListNode or TreeNode. So rather than writing a function that works on only TreeNode objects, we should write a function that works on anything that has a left and a right property. #157 offers some examples of how to do this.