feat: grouped export and import

[stefanbildl]

Export or import tasks grouped by desc

I found the grouped task issue #56 interesting and wanted to give it a try.

First of all, I preserved the standard json output format. So everything is backwards compatible.

I added a new flag -g (--grouped) to the to-json command that exports the tasks grouped by title.
If you restart a task with r in the UI or reuse an existing name, the grouped json output collects the ones with the same name.

The from-json import command works with both formats. It automatically detects which one was used.

❯ tt to-json -p example -g
{
  "task 1": [
    {
      "start": "2026-04-23T17:39:10+02:00",
      "end": "2026-04-23T17:39:22+02:00"
    },
    {
      "start": "2026-04-23T17:39:01+02:00",
      "end": "2026-04-23T17:39:08+02:00"
    }
  ],
  "task 2": [
    {
      "start": "2026-04-23T17:39:08+02:00",
      "end": "2026-04-23T17:39:10+02:00"
    }
  ],
  "task 3": [
    {
      "start": "2026-04-23T17:39:34+02:00",
      "end": "2026-04-23T17:39:39+02:00"
    },
    {
      "start": "2026-04-23T17:39:27+02:00",
      "end": "2026-04-23T17:39:33+02:00"
    },
    {
      "start": "2026-04-23T17:39:22+02:00",
      "end": "2026-04-23T17:39:24+02:00"
    }
  ]
}`

I preferred this format because the solution proposed by #56 (comment) was a bit too complex for my taste.

I hope you like it :)