Issue Type: Feature Request

Right now only active text editor can be tracked with window.activeTextEditor. Though users can focus other UI elements, like integrated terminals, there is no way to tell when user switches focus from the text editor to the integrated terminal, or from one terminal to another.

Because of that user tracking in our extension cannot tell when user has changed focus to an integrated terminal, or switched active terminals.

We'll need something like window.activeTextEditor, but more generic, e.g.
activeWindow: TextEditor | Terminal | .... | undefined