Core/CreatureAI: Implement an OnSpellClick hook instead of the ugly way that uses DoAction with EVENT_SPELLCLICK.

Core/SmartAI: Implement SMART_EVENT_ON_SPELLCLICK. Parameter is the clicker.

If someone could update the wiki accordingly for SAI I would appreciate that. Thanks to @Shauren for some help!

src/server/game/Entities/Unit/Unit.cpp

@@ -16935,7 +16935,7 @@ bool Unit::HandleSpellClick(Unit* clicker, int8 seatId)
 
     Creature* creature = ToCreature();
     if (creature && creature->IsAIEnabled)
-        creature->AI()->DoAction(EVENT_SPELLCLICK);
+        creature->AI()->OnSpellClick(clicker);
 
     return true;
 }