Feature: split pane support

[starpit]

1. it'd be nice to have split panes.
2. we could use these as a better host for the watch panes
3. we need to clean up and declare types for the Command lifecycle events; this will be important with split terminals

[starpit]

Architecture

• ScrollableTerminal will encapsulate the splits
• It will implement a Tab facade so that command lifecycle events can be directed to a particular split
• This shouldn't require many changes elsewhere. But... we will need to improve the event substrate. Right now, we are using the untyped eventChannelUnsafe. We should use this opportunity to type it up. The types will be in events.ts, where eventBus is normally situated, but updated with newly-typed event channels related to the lifecycle of command execution.