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)

Start by locating the plugin's existing stage implementations and entry points for stage progression. Confirm how a KillMobs stage should track the required mob count and when progression occurs, then verify that the player advances only after the requested number of mobs is killed.

Written by the indexing model from the issue text.