[8929] Implement gossip scripts to be used with gossip_menu_option.action_script_id

Note that script is implemented for GOSSIP_OPTION_GOSSIP only (as not expected needed for other gossip options).
Also add a few more startup checks for LoadGossipMenuItems with check for basic errors in fields

src/game/World.cpp

@@ -1402,6 +1402,9 @@ void World::SetInitialWorldSettings()
     sLog.outString( "Loading Npc Text Id..." );
     sObjectMgr.LoadNpcTextId();                                 // must be after load Creature and NpcText
 
+    sLog.outString( "Loading Gossip scripts..." );
+    sObjectMgr.LoadGossipScripts();                             // must be before gossip menu options
+
     sLog.outString( "Loading Gossip menus..." );
     sObjectMgr.LoadGossipMenu();