Add completed timestamp to TodoItem

[karwosts]

I would like to request if I can add an optional completed timestamp to TodoItem denoting the time that that item was last marked completed.

Working on building out recurring chores in my local todo list via automations, and it would be helpful to know when an item was completed for helping the rescheduling logic. Right now I'm just munging this data into the description field via an automation every time a task is marked completed, but having native support would be much simpler.

Also it could just be displayed as informative when simply viewing the task list, someone may want to see when an item was last completed.

Could be used for local_todo lists (the ical format has a completed timestamp property), google tasks also has a completed timestamp for items.