LibJS: Implement Iterator.prototype.map

This uses a new Iterator type called IteratorHelper. This does not
implement IteratorHelper.prototype.return as that relies on generator
objects (i.e. the internal slots of JS::GeneratorObject), which are not
hooked up here.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -143,6 +143,8 @@ set(SOURCES
     Runtime/Intrinsics.cpp
     Runtime/Iterator.cpp
     Runtime/IteratorConstructor.cpp
+    Runtime/IteratorHelper.cpp
+    Runtime/IteratorHelperPrototype.cpp
     Runtime/IteratorOperations.cpp
     Runtime/IteratorPrototype.cpp
     Runtime/JSONObject.cpp