Allow completing monster killing mission by killing the fusing monster when the mission target is fused

[Qrox]

Summary

SUMMARY: Bugfixes "Allow completing monster killing mission by killing the fusing monster when the mission target is fused"

Purpose of change

Fixes #46283.

Describe the solution

1. Fix a premature return caused by the death_drops flag, so mission status is correctly updated when a monster dies when death_drops is false.
2. When a monster you are asked to kill gets fused with another monster, you'll have to kill the fusing monster to complete the mission. However, if you are asked to find the monster, the mission counts as failed instead.
3. When a mission monster is fused into another monster, a message will be displayed hinting that the fusing monster needs to be killed. The fusing monster's description will mention the fused monster and the name will start with an asterisk so the player can more easily spot the monster.

Describe alternatives you've considered

Go easy on the player and count fused monster as killed.
Make a mission specific flag that counts fused monster as killed.

Testing

Added the kill zombie mom mission to an NPC via the debug menu, teleported to the zombie mom, spawned a dissoluted devourer, and waited until it fuses with the zombie mom. The mission still required killing the devourer. Bashed the devourer to death, and the mission was then updated.