Add support for dependencies in tasks

[tionis]

RFC9253 introduces some additional RELTYPEs to the standard, one of them being:

RELTYPE=DEPENDS-ON:
This indicates that the current calendar component depends on the referenced calendar component in some manner. For example, a task may be blocked waiting on the other, referenced, task.

It would be nice to have support for that, as tasks often do have blockers and nearly no to-do apps consider this problem.
I'm not sure at the moment how that should look or work exactly, though.

[tionis]

After thinking about it I think it would be nice to be able to specify a list of tasks that need to be finished before in the edit panel (e.g. by specifying it's id, perhaps later an fzf style interactive selector?)
Show and list would also have to show dependencies somehow e.g. via something like @depends_on(1, 2) or whatever fits the current style best.

The main benefit would then be an integration into todo list --startable as only tasks for which all dependencies are done would be shown here (unless the task is overdue maybe?)

[tionis]

Ups, wrong button.
Could some please reopen this, I don't have permission to do it.
Perhaps @WhyNotHugo ?