Summary:
A pending change revamps a lot of how ESLint, Prettier, and Jest are configured in the repo, along with moving the world to TypeScript (and adding tsc usage).
That still needs some work to split up, but this change adds types used in our unit tests that I found were missing.
Differential Revision: D45506781
fbshipit-source-id: e3ee49ebef1b7d28c220ab08fd5740e31c8632cb
