KillMobs Stage

Implement a stage that requires the player to kill a certain amount of mobs before being incremented. This is to further the potential use of the plugin as a quest plugin (and demonstration of it as such)

首先定位外掛程式中現有的 stage 實作以及 stage progression 的進入點。確認 KillMobs stage 應如何追蹤所需的 mob 數量以及何時發生 progression，然後驗證玩家只有在擊殺所要求數量的 mob 後才會前進。

由索引模型根據 Issue 內容生成。