LibJS: Implement a very hackish "arguments" object

We now lazily create an "arguments" array inside functions when code
tries to access it.

This doesn't follow the spec at all but still covers a lot of the
basic uses of arguments, i.e "arguments.length" and "arguments[n]"

Libraries/LibJS/Tests/arguments-object.js

@@ -0,0 +1,16 @@
+test("basic arguments object", () => {
+    function foo() {
+        return arguments.length;
+    }
+    expect(foo()).toBe(0);
+    expect(foo(1)).toBe(1);
+    expect(foo(1, 2)).toBe(2);
+    expect(foo(1, 2, 3)).toBe(3);
+
+    function bar() {
+        return arguments[1];
+    }
+    expect(bar("hello", "friends", ":^)")).toBe("friends");
+    expect(bar("hello")).toBe(undefined);
+});
+