[Breaking] [Behaviours/1] Navigator refactor: turn NavigatorCommands into BehaviorTree Tasks

Since we are going to rework the Navigator API anyways, urgently need to rework the Navigator to work with BehaviorTrees. But BehaviorTrees basically remove the need for a Navigator-- all the NavigatorCommands will just become Task nodes in a BehaviorTree.

Turn them into Tasks with stubbed out tick() functions