Add `transitioning` to `<Modal>` slots prop

[typed-sigterm]

Description

teansitioning = true if the modal is fading in/out

Additional context

No response

[J-Michalek]

I don't think RekaUI exposes nor tracks the animation state. Perhaps you could implement it using animationstart and animationend events combined with useEventListener from VueUse?

https://developer.mozilla.org/en-US/docs/Web/API/Element/animationstart_event
https://developer.mozilla.org/en-US/docs/Web/API/Element/animationend_event

[typed-sigterm]

I just see after-enter after-leave events work, so I created #4760