📚 Project: Add type predicate appetizer to modifiers-of-the-types

[JoshuaKGoldberg]

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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. 🙂