Enable unicorn/prefer-at unconditionally (PR 15014 follow-up)

Now that Node.js version 18 is required, we should be able to use `Array.prototype.at()` everywhere in the code-base.

external/builder/preprocessor2.js

@@ -244,8 +244,8 @@ function postprocessNode(ctx, node) {
       const block = node.body;
       if (
         block.body.length > 0 &&
-        block.body[block.body.length - 1].type === "ReturnStatement" &&
-        !block.body[block.body.length - 1].argument
+        block.body.at(-1).type === "ReturnStatement" &&
+        !block.body.at(-1).argument
       ) {
         // Function body ends with return without arg -- removing it.
         block.body.pop();