Feature Request: Reset task nubmer

**Is your feature request related to a problem? Please describe.**
For example, If I created 3 tasks, they are:
```
1. task 1
2. task 2
3. task 3
```

Then, I delete @3, and create a new task 4. The new task will be created as number 4
```
1. task 1
2. task 2
4. task 4
```

I can imagine if I use taskbook for a long time. I will have maybe only 1 task in the list, but its number could be @99999, which is a bit inconvenience to type more digits.

**Describe the solution you'd like**
Is it possible to separate task id (which is unique in json) and task number (showing in the list)? And task number is recyclable, for the case above, it should show:
```
1. task 1
2. task 2
3. task 4
```

Start with taskbook's CLI task creation and deletion flow, then trace how task numbers and unique IDs are stored in JSON. Determine how display numbering can be separated from persistent identity without breaking existing tasks. Done means deleted numbers are reused in the list while each task keeps a unique JSON identity.

Written by the indexing model from the issue text.