introduce EnterStatePolicy

[pierremrtn]

Introduce ReEnterStatePolicy object that lets users control state re-entering conditions:

enum ReEnterStatePolicy {
    always,
    notIdentical,
    notEqual,
    never,
}

This allows users to control whether the machine considers a transition from the same state as a new transition or not (and re-triggering onEnter / onExit callbacks).

• always: Always enter the state when a transition returns the state, regardless of whether the current state is already the same. This allows for mutable states.

• notIdentical: Enter the state when a transition returns the state and if the current state is not identical to the new state (using identical for comparison).

• notEqual: Enter the state when a transition returns the state and if the current state is not equal to the new state (using the == operator).

• never: Prevent re-entering the state if a transition returns the same state as the current one. This allows for disabling the re-trigger of parent state onEnter / onExit callbacks.