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/IteratorConstructor.h

@@ -24,6 +24,8 @@ private:
     explicit IteratorConstructor(Realm&);
 
     virtual bool has_constructor() const override { return true; }
+
+    JS_DECLARE_NATIVE_FUNCTION(from);
 };
 
 }