Add a new EOC event, `npc_changes_opinion`

[MNG-cataclysm]

Summary

Content "Add a new EOC event, npc_changes_opinion."

Purpose of change

Add a new EOC trigger, npc_changes_opinion. This will allow an EOC to run whenever an NPC's opinion values change.

Describe the solution

Add a new EOC event that can trigger whenever opinion, trading, or math is used to change an NPC's opinion. This will also be able to track what opinion changed, by how much, and for what reason it did so. EOC's will be able to trigger off of this, and perform a variety of effects.

Describe alternatives you've considered

Not doing this.

Testing

To be done.

Additional context

N/A at the moment.