Use @types/eslint-config-prettier

[JoshuaKGoldberg]

🚀 Feature Request

Following #412: now that DefinitelyTyped/DefinitelyTyped#44245 was merged, we can use it.

Existing Behavior

src/typings/eslint-config-prettier.d.ts

Change Proposal

npm i @types/eslint-config-prettier --save-dev