add support for todo.txt format

[cizmazia]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.
• I optionally donated to support the Fossify mission.

Feature description

Add a setting to use the todo.txt format instead of JSON for saving/exporting task lists. (Even partial support of the format would be great!)

Why do you want this feature?

It would enable to use a plain text editor to modify the task lists. Task list files can be synchronized to other devices using Syncthing.

The only existing alternative app SimpleTask is not actively developed any longer.

Additional information

No response