Add TypeScript goodies for traversing binary trees

[miorel]

Tested on Leetcode:

• Preorder: https://leetcode.com/problems/binary-tree-preorder-traversal/submissions/1316869104/
• Inorder: https://leetcode.com/problems/binary-tree-inorder-traversal/submissions/1316866526/
• Postorder: https://leetcode.com/problems/binary-tree-postorder-traversal/submissions/1316867885/

[github-actions]

PR Health Report

Last checked commit 591f04f.

• npx prettier --write .github .vscode && ! (git status --porcelain | grep .): ✅
• (cd tools && yarn format && ! (git status --porcelain | grep .)): ✅
• (cd tools && yarn lint): ✅
• (cd tools && yarn typecheck): ✅
• (cd tools && yarn test): ✅
• (cd tools/adventure-pack && yarn build-app): ✅

[elimanzo]

Looks good. can't wait to see level order :)

[miorel]

Looks good. can't wait to see level order :)

Created #161 as we discussed 😄