LibJS: Implement proper Iterator records

Instead of using plain objects as Iterator records, causes confusion
about the object itself actually being its [[Iterator]] slot, and
requires non-standard type conversion shenanigans fpr the [[NextValue]]
and [[Done]] internal slots,  implement a proper Iterator record struct
and use it throughout.

Also annotate the remaining Iterator AOs with spec comments while we're
here.

Userland/Libraries/LibJS/Runtime/Iterator.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+// Iterator Record
+struct Iterator {
+    Object* iterator { nullptr }; // [[Iterator]]
+    Value next_method;            // [[NextMethod]]
+    bool done { false };          // [[Done]]
+};
+
+}