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/Forward.h

@@ -69,6 +69,7 @@
 #define JS_ENUMERATE_ITERATOR_PROTOTYPES          \
     __JS_ENUMERATE(Iterator, iterator)            \
     __JS_ENUMERATE(ArrayIterator, array_iterator) \
+    __JS_ENUMERATE(SetIterator, set_iterator)     \
     __JS_ENUMERATE(StringIterator, string_iterator)
 
 #define JS_ENUMERATE_BUILTIN_TYPES \