LibJS: Implement Iterator.from and the WrapForValidIteratorPrototype

Iterator.from creates an Iterator from either an existing iterator or
an iterator-like object. In the latter case, it sets the prototype of
the returned iterator to WrapForValidIteratorPrototype to wrap around
the iterator-like object's iteration methods.

Userland/Libraries/LibJS/Runtime/Iterator.h

@@ -35,4 +35,7 @@ private:
     IteratorRecord m_iterated; // [[Iterated]]
 };
 
+ThrowCompletionOr<IteratorRecord> get_iterator_direct(VM&, Object&);
+ThrowCompletionOr<IteratorRecord> get_iterator_flattenable(VM&, Value);
+
 }