Type alias Runner

type definition for when a signal's update function run is called by the signal update propagator propagateSignalUpdate inside of Context.
the return value should indicate whether this signal has:

• updated (SignalUpdateStatus.UPDATED === 1), and therefore propagate to its observers to also run
• unchanged (SignalUpdateStatus.UNCHANGED === 0), and therefore not propagate to its observers
• aborted (SignalUpdateStatus.ABORTED === -1), and therefore force each of its observers to also become non propagating and inherit the SignalUpdateStatus.ABORTED status