This is small clean-up pull request which removes some functions that were unnecessarily declared to return promise/thenable and also adds a couple of await when we respond to selection or tab change events from the VS Code window. It's possible that this will solve #308, but since I can't reproduce it, I can't be certain.

There is a more ambitious change that would involve making the core execute function of each mode async and awaiting it in the input handler, but I suspect that might make the extension feel a bit laggy, so I'd rather not do that unless we're sure it's going to make a difference.