LibJS: Add the SetIterator built-in and Set.prototype.{values, entries}

While this implementation should be complete it is based on HashTable's
iterator, which currently follows bucket-order instead of the required
insertion order. This can be simply fixed by replacing the underlying
HashTable member in Set with an enhanced one that maintains a linked
list in insertion order.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -78,6 +78,8 @@ set(SOURCES
     Runtime/ScriptFunction.cpp
     Runtime/Set.cpp
     Runtime/SetConstructor.cpp
+    Runtime/SetIterator.cpp
+    Runtime/SetIteratorPrototype.cpp
     Runtime/SetPrototype.cpp
     Runtime/Shape.cpp
     Runtime/StringConstructor.cpp