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();
+});

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

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

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

@@ -0,0 +1,33 @@
+// Update when more typed arrays get added
+const TYPED_ARRAYS = [
+    Uint8Array,
+    Uint16Array,
+    Uint32Array,
+    Int8Array,
+    Int16Array,
+    Int32Array,
+    Float32Array,
+    Float64Array,
+];
+
+test("basic functionality", () => {
+    TYPED_ARRAYS.forEach(T => {
+        expect(T.prototype.at).toHaveLength(1);
+
+        const typedArray = new T(3);
+        typedArray[0] = 1;
+        typedArray[1] = 2;
+        typedArray[2] = 3;
+
+        expect(typedArray.at(0)).toBe(1);
+        expect(typedArray.at(1)).toBe(2);
+        expect(typedArray.at(2)).toBe(3);
+        expect(typedArray.at(3)).toBeUndefined();
+        expect(typedArray.at(Infinity)).toBeUndefined();
+        expect(typedArray.at(-1)).toBe(3);
+        expect(typedArray.at(-2)).toBe(2);
+        expect(typedArray.at(-3)).toBe(1);
+        expect(typedArray.at(-4)).toBeUndefined();
+        expect(typedArray.at(-Infinity)).toBeUndefined();
+    });
+});