Core/Scripts: dynamic_spawning follow-up, I had forgotten JustRespawned existed.

- Rename JustRespawned to JustAppeared, which better matches its behavior anyway.
- Properly invoke JustAppeared for new (re-)spawns - fixes #20111.
- Fix Thaddius script to work with dynamic_spawning (mostly unrelated to the above) - Feugen/Stalagg should really be a summon group, but I don't have time to fix that right now.
- Fix default value for DynamicEscortNPC to match worldserver.conf.dist.

src/server/game/AI/SmartScripts/SmartAI.cpp

@@ -566,7 +566,7 @@ bool SmartAI::AssistPlayerInCombatAgainst(Unit* who)
     return false;
 }
 
-void SmartAI::JustRespawned()
+void SmartAI::JustAppeared()
 {
     mDespawnTime = 0;
     mDespawnState = 0;

src/server/game/AI/SmartScripts/SmartAI.h

@@ -71,7 +71,7 @@ class TC_GAME_API SmartAI : public CreatureAI
         bool IsEscortInvokerInRange();
 
         // Called when creature is spawned or respawned
-        void JustRespawned() override;
+        void JustAppeared() override;
 
         // Called at reaching home after evade, InitializeAI(), EnterEvadeMode() for resetting variables
         void JustReachedHome() override;