refactor: 可読性のため一部でArray.prototype.atを使うように (#11274)

* refactor: `Array.prototype.at`を使うように

* fixup! refactor: `Array.prototype.at`を使うように

packages/backend/src/misc/prelude/array.ts

@@ -67,8 +67,9 @@ export function maximum(xs: number[]): number {
 export function groupBy<T>(f: EndoRelation<T>, xs: T[]): T[][] {
 	const groups = [] as T[][];
 	for (const x of xs) {
-		if (groups.length !== 0 && f(groups[groups.length - 1][0], x)) {
-			groups[groups.length - 1].push(x);
+		const lastGroup = groups.at(-1);
+		if (lastGroup !== undefined && f(lastGroup[0], x)) {
+			lastGroup.push(x);
 		} else {
 			groups.push([x]);
 		}