Quests: Implement KILL_NPC mission type

[elizabethli31]

We were able to incorporate quests into NPCs, but right now the player can only talk to them for task completion. Perhaps this can be how the KILL_NPC mission type is implemented, since that is currently used in some places in our tests but is not actually fully supported (missions of that type aren't automatically managed by Chiventure like other mission types). This could require working with the battle team, so missions can have fighting