LibJS: Implement 'Relative Indexing Method' proposal (.at())

Still stage 3, but already implemented in major engines and unlikely to
change - there isn't much to change here anyway. :^)

See:

- https://github.com/tc39/proposal-relative-indexing-method
- https://tc39.es/proposal-relative-indexing-method/
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.at.js

@@ -0,0 +1,15 @@
+test("basic functionality", () => {
+    expect(Array.prototype.at).toHaveLength(1);
+
+    const array = ["a", "b", "c"];
+    expect(array.at(0)).toBe("a");
+    expect(array.at(1)).toBe("b");
+    expect(array.at(2)).toBe("c");
+    expect(array.at(3)).toBeUndefined();
+    expect(array.at(Infinity)).toBeUndefined();
+    expect(array.at(-1)).toBe("c");
+    expect(array.at(-2)).toBe("b");
+    expect(array.at(-3)).toBe("a");
+    expect(array.at(-4)).toBeUndefined();
+    expect(array.at(-Infinity)).toBeUndefined();
+});