Clean up handling of lifecycle phases

[tschneidereit]

@guybedford pointed out in a comment that the name for a newly introduced phase of a JS app's lifecycle is a bit off.

Rather than addressing that in-place, I'm filing this issue to track overhauling our overall management of these phases. We have a spattering of different things, from wizening to initialized to runtime, etc. I think we should clean all those up, and introduce a state-transitioning concept, capturing everything from "the runtime has been compiled, but no part of it ever ran" to "the JS runtime has been initialized, but nothing else", to "some code is currently being evaluated", etc, all the way to either "the main execution phase has finished, but could be re-run" or "the main execution phase has finished, and no more code must be run in this instance, because no cleanup happened".