Add support for 'strict' targets on the Agent interface

[pipex]

A Target in Mahler is by default 'relative', meaning that only property
changes and additions should be considered when comparing current and
target states for planning. Property deletion need to be done explicitely
via the UNDEFINED symbol. This allows a cleaner interface for for
defining system targets and allows the system state to have additional properties
than the target.

A 'relative' target is the opposite to a 'strict' (or absolute) target, where what is passed to
the planner/agent describes exactly the desired state of the system is.

This PR introduces the StrictTarget type and the seekStrict method
on the Agent interface. This allows users some additional flexibility
to specify an absolute state of the final system.

Example: let's say we are modelling the state of two variables x and y.

Given the current state {x: 0}, the target state {y: 1} means that the
planner needs to only to find a task that can create the variable y and increase its
value to 1. The final expected state should be {x: 0, y:1} (assuming nothing else changes x).

If the goal was to remove the variable x at the same time that variable y is introduced, the
relative target would need to be {x: UNDEFINED, y: 1}

The equivalent strict target in this case is just {y: 1}.

Change-type: minor

[pipex]

I self-certify!