Assisted organ sprites, and assisted brains (#19087)

This PR adds several sprites for assisted organs, which previously
lacked any sprites at all. In addition, this adds an assisted brain,
which is now available to pick in character creation. Long-term, I'd
like to look into adding assisted implants into the medical gameplay
loop, but for now these are character-creation only. "Mechanical"
prosthetic brains are not possible, and have been excised entirely.

### Asset Licenses
The following assets that **have not** been created by myself are
included in this PR:

| Path | Original Author | License |
| --- | --- | --- |
| icons/obj/organs/organs.dmi | Mechoid (Polaris) | CC-BY-SA |

---------

Co-authored-by: Cody Brittain <cbrittain10@live.com>

code/modules/organs/internal/_internal.dm

@@ -91,6 +91,10 @@
 
 			robotize_type = company
 
+/obj/item/organ/internal/mechassist()
+	..()
+	icon_state = "[initial(icon_state)]-assisted"
+
 /obj/item/organ/internal/proc/getToxLoss()
 	if(BP_IS_ROBOTIC(src))
 		return damage * 0.5

code/modules/organs/internal/brain.dm

@@ -3,6 +3,7 @@
 	desc = "A piece of juicy meat found in a person's head."
 	organ_tag = BP_BRAIN
 	parent_organ = BP_HEAD
+	possible_modifications = list ("Normal","Assisted")
 	vital = TRUE
 	icon_state = "brain"
 	force = 1