Add a game end trigger for negotiation and combat

[RageLeague]

I am modding the game and I wanted to add a modifier that says "gain x shills at the end of the game". When I look up negotiation_def.lua I didn't find an event trigger for when the game is about to end, so I have to use a workaround. I don't want to modify the negotiation engine directly because it might cause many problems when the devs change it. Adding a game end trigger can make modding easier, and it creates more rooms for exploration of mechanics.

[rooks]

I'll add this in a coming patch.