Basic mobs can emote when they get a new target (#76654)

## About The Pull Request


![image](https://github.com/tgstation/tgstation/assets/7483112/c16adf49-b075-42ee-a5e1-d998e0e77227)

Fixes #76624
Restores lost functionality for a mob to play an emote when it acquires
a new target.
Also I added a generic blackboard key change signal which could
potentially have other uses in the future.

## Why It's Good For The Game

It's good feedback to know when a mob has chosen to target something.

## Changelog

🆑
fix: Carp, Trees, and Festivus Poles will once more sometimes emote at
you if they think you are looking at them the wrong way, before they
come over to beat you up.
/🆑

code/modules/mob/living/basic/tree.dm

@@ -56,6 +56,7 @@
 	. = ..()
 	AddElement(/datum/element/swabable, CELL_LINE_TABLE_PINE, CELL_VIRUS_TABLE_GENERIC_MOB, 1, 5)
 	AddElement(/datum/element/death_drops, list(/obj/item/stack/sheet/mineral/wood))
+	AddComponent(/datum/component/aggro_emote, emote_list = string_list(list("growls")), emote_chance = 20)
 
 /mob/living/basic/tree/Life(seconds_per_tick = SSMOBS_DT, times_fired)
 	..()