Print changed "duration" in "task info" when start/end are changed

When start/end times of a completed task are changed, the CLI output of task info just prints a statement like:

```
End changed from 'Sat 26 Jun 2021 15:15:54' to 'Fri 25 Jun 2021 05:00:00'.
```

Given duration was printed when task was "done", that duration is now stale... So the suggestion in this ticket is to print duration too if duration has changed.

```
End changed from 'Sat 26 Jun 2021 15:15:54' to 'Fri 25 Jun 2021 05:00:00'.
(duration changed from: old duration to new duration)
```

Same goes with editing the start time of completed task. Currently if I modify start of a task, it just prints:
```
Start set to 'Fri 25 Jun 2021 03:00:00'.
```

The suggestion is to print:

```
Start set to 'Fri 25 Jun 2021 03:00:00'.
(duration changed from: old duration to new duration)
```

> Perhaps task warrior can print duration only once at the end of a start - done epoch. This will require taskwarrior to remove string (duration:) from the current completed epoch before printing a fresh one in the info line of the current edit to the task. In that case, instead of printing "(duration changed from:...", it would just print "(duration:)" like below. You can think which one makes more sense for task warrior. Thanks :)

```
Start set to 'Fri 25 Jun 2021 03:00:00'. (duration: )
```

> This can make it very easy to read task info and calculate how much total time was doing a task. Let's say if a task had 5 start-done epochs, then the user will see just 5 (duration:) fields in task info. Which itself can perhaps have a different color group so it stands out from rest of the info.