addPrettierExtensions doesn't need to be async

[JoshuaKGoldberg]

Overview

Quick little tech debt point I just noticed while investigating #729: although other logic around rule extensions called by summarizePackageRules does need to be async, addPrettierExtensions itself makes no asynchronous calls. It can be changed to a non-async function.

The name also is a relic, I think, of an implementation draft that is no longer accurate. Maybe... checkPrettierExtension would be a better name for it? It doesn't add any extensions; it just checks whether the Prettier config should be extended from.