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)

首先定位插件中现有的 stage 实现以及 stage progression 的入口点。确认 KillMobs stage 应如何跟踪所需的 mob 数量以及何时发生 progression，然后验证玩家只有在击杀所请求数量的 mob 后才会推进。

由索引模型根据 Issue 内容生成。