From one of the SketchUp developers (jbacus) over in the SketchUp forums:
"I am keen to support end-user development in SketchUp for Web in the future, but as you guys have seen me say several times already, this is a tough project that isn’t going to come soon. We’re much more able to implement the sort of features that are most commonly accessed through an extension right in the core application. That’s the path we’re on for now."
https://forums.sketchup.com/t/what-s-up-with-sketchup-make/55219/906