Core/Scripts: Add new SpellScript hook OnEffectSuccessfulDispel

Adds SpellScript hook to execute scripts after succesful dispels, example could be warlock pets Devour Magic, which should only trigger when succesfully dispelling an aura.
Core/Scripts: Rewrite Devour Magic
Rewrite Devour Magic to use new spellscript hook OnEffectSuccessfulDispel.
Merge pull request #13751 from Unholychick/6.0.3

Core/Scripts: Add new SpellScript hook