Enable deletion of partial intervals/ranges

Somewhat related to #257, I would like to have a simple way to delete part of an interval - e.g. if I took a break.

I think the most intuitive way would be to allow delete to take a range, e.g.
```sh
timew delete 1400 - 1430
```
This would automatically split the intervals at either end if necessary

The ways I see to currently do it is to either track a temporary interval with the `:adjust` hint and then delete that, or split two times and then delete the interval in between.

Start by tracing the existing `timew delete` command and the interval-splitting behavior mentioned in the issue. The work is done when a range such as `1400 - 1430` deletes that portion and automatically splits intervals at either end when necessary.

Written by the indexing model from the issue text.