Reduce side effects of SeparationRayShape2D.slide_on_slope

[WesleyClements]

I've been running into some issues with what seems to me to be an unintuitive design choice for the behavior of SeparationRayShape2D. From what I can see in the implementation, SeparationRayShape2D is intended to be used for depenetration and is excluded from the collision detection that would interrupt motion. It seems that the intended way to allow a SeparationRayShape2D to be used to interrupt motion is by enabling SeparationRayShape2D.slide_on_slope. Unfortunately SeparationRayShape2D.slide_on_slope additionally changes the collision response to not separate exclusively along the ray. You could alternatively set PhysicsTestMotionParameters2D.collide_separation_ray to true but that isn't accessible from inside of CharacterBody2D. Based on the way it was implemented, it feels like PhysicsTestMotionParameters2D.collide_separation_ray was added to explicitly override this default behavior because the implementation caused problems for floor snapping in CharacterBody2D.

With all that in mind I want to pose a couple questions.

1. Why is it necessary to ever exclude SeparationRayShape2D from collision detection that would interrupt motion? I do see that the implementation isn't really robust enough to account for motion accurately. Is that the reason? As far as I can see, SeparationRayShape2D would still separate along the direction of the ray even if it interrupted motion.
2. If it is truly necessary to be able to exclude SeparationRayShape2D in this way, wouldn't it be better to change SeparationRayShape2D.slide_on_slope to an Enum of the different possibly desired behaviors rather than making assumptions about usage? The names could be better but something like:

enum SeparationRayCollisionResponse {
  DEPENETRATION_IN_DIRECTION, # same as slide_on_slope == false currently
  COLLISION_IN_DIRECTION,
  DEPENETRATION,
  COLLISION # same as slide_on_slope == true currently
}

Generally it is unintuitive for me that SeparationRayShape2D arbitrarily diverges in behavior from other Shape2D (especially as the default behavior).