Deadline Support for sorting and/or process

[stacksjb]

Is your feature request related to a problem? Please describe.
Todoist has recently added their Deadlines feature (https://todoist.com/help/articles/introduction-to-deadlines-uMqbSLM6U and https://todoist.com/help/articles/whats-new-WV4aRXKsN#deadlines)

Describe the solution you'd like
Add deadline support
I see two areas this could be added:

1. Add as a value used in sorting (sort upcoming/due in next items OR items with a due date ahead of anything else)
2. Add an option to set a deadline when processing or labelling (probably as part of the "pioritize", "Timebox", or schedule settings, maybe as as a submenu for scheduling)

Describe alternatives you've considered
None - new feature

Additional context
I don't think anything around deadline needs to be added, as the task actions still happen around due dates, and deadlines are intended for once due tasks (not recurring) that shouldnt change.