chore: add type comment to exported plugin (#488)

Author: JoshuaKGoldberg

lib/index.js

@@ -42,6 +42,7 @@ const allRules = Object.fromEntries(
     ]),
 );
 
+/** @type {import("eslint").ESLint.Plugin} */
 const plugin = {
   meta: {
     name: packageMetadata.name,