refactor dynamic scene selection to be more user-friendly

[Logodaedalus]

This currently lives in Coordinator.getNextScene()

Right now you'd have to manually insert your logic in there on a per-project basis. It'd be better if:

1. We establish as a convention that State variables with the "sys_" prefix are for StoryAssembler (add to documentation)
2. We automatically set "sys_nextScene" in StoryAssembler code to the scene id of the next one in the array before running any fragments (by default it goes to the next one in the list)
3. In Coordinator.getNextScene(), change it so that it uses that value for next scene
4. Put in docs that the way you conditionally control that is by setting "sys_nextScene"