Spike: Push SuperEditor as far as possible

[matthew-carroll]

SuperEditor solves a lot of layout and selection problems. Take SuperEditor as far as possible, as the foundation for Inception's code layout, styling, and selection.

I expect that at some point SuperEditor will not meet the needs of Inception. At that point, consider building a line-oriented document layout implementation. Update SuperEditor so that other pieces of the document puzzle or composable for our needs, e.g., content selection, auto-scrolling, and overlay controls.