feat(Core/Scripting): Implement OnBeforeCreatureSelectLevel() hook (#17391)

* Add `OnBeforeCreatureSelectLevel()` hook
- allows modules to alter the level of a creature just before it is set

* Fix missing class name.

* Hook should catch even if the level isn't being changed.

* Trim whitespace.

* Revert "Trim whitespace."

This reverts commit 76768d85c0.

* Fix whitespace

src/server/game/Scripting/ScriptMgr.h

@@ -555,6 +555,9 @@ public:
     // Called from End of Creature Update.
     virtual void OnAllCreatureUpdate(Creature* /*creature*/, uint32 /*diff*/) { }
 
+    // Called just before the level of the creature is set.
+    virtual void OnBeforeCreatureSelectLevel(const CreatureTemplate* /*cinfo*/, Creature* /*creature*/, uint8& /*level*/) { }
+
     // Called from End of Creature SelectLevel.
     virtual void Creature_SelectLevel(const CreatureTemplate* /*cinfo*/, Creature* /*creature*/) { }
 
@@ -2555,6 +2558,7 @@ public: /* MovementHandlerScript */
 public: /* AllCreatureScript */
     //listener function (OnAllCreatureUpdate) is called by OnCreatureUpdate
     //void OnAllCreatureUpdate(Creature* creature, uint32 diff);
+    void OnBeforeCreatureSelectLevel(const CreatureTemplate* cinfo, Creature* creature, uint8& level);
     void Creature_SelectLevel(const CreatureTemplate* cinfo, Creature* creature);
     void OnCreatureSaveToDB(Creature* creature);