Allows using the fancy-ctrl-z functionality of toggling between the last
two suspended jobs (`%+` and `%-`, respectively) even if there are more
than two jobs suspended.
Made the whole implementation more robust, especially in case Ctrl-Z is
pressed with an existing non-empty prompt buffer.
Fixed a bug where jobs with multiple different working directories would
cause the back-and-forth functionality to not work.
Added a feature to `fancy-ctrl-z` which, if and only if there are
precisely two backgrounded jobs, switches between them (using `fg %-`)
when bringing back something to the foreground.