fix: award killer their own kill-bonus exp on monster death

- Uncomment Creature.HandleKillerBonus to grant the killer 5%-of-MaxHP bonus

exp (color-adjusted via AdjustCharExperienceGain) on top of the

damage exp from the final blow

- Rewire it into Creature.Die after the team-share loop so it runs for

both solo and team killers, and for both melee and magic kills

- Send a "You got X exp points by killing a monster." sysmsg mirroring

the original STR_KILL_EXPERIENCE notice