📚 Project: Add type predicate appetizer to Type Modifiers

[JoshuaKGoldberg]

Overview

Filing a todo for myself: I plan on adding a third appetizer under https://www.learningtypescript.com/type-modifiers/modifiers-of-the-types that exercises type predicates.

This isn't accepting PRs, just using the issue tracker so I don't forget. 🙂