Support to change default transition class

[gjm]

Rationale:

If I have a model with 5 possible states, without transition restrictions (see #153), but I want to use a custom transition that is identical between all of them, I have to create a custom transition class for every possible transition.

It should be possible, in the base State to be able to specify a default transition class (would fallback to the current implementation if none where specified). I would suggest something like this in the base State class:

protected function resolveDefaultTransition(string $from, string $to): Transition
{
    ...
}

This way each (base) state could provide it's own default transition class.