Prometheans can now change hair transparency separate from body transparency. (#5832)

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## About The Pull Request

Title.
Hair transparency is now configurable from an opacity of 100-255 for
prometheans.
However, body transparency is still a toggle. This is because apparently
the transparent body is an actual _sprite_ rather than just changing the
transparency of the sprite, and I don't want to bother changing that
right now. Maybe in a later PR I will.

## Why It's Good For The Game

Customization good.

## Changelog

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🆑
qol: Prommies can now change their hair transparency separate from body
transparency.
/🆑

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code/modules/organs/external/species/slime.dm

@@ -67,3 +67,4 @@
 	encased = 0
 	spread_dam = 1
 	transparent = 1
+	hair_opacity = 160

code/modules/organs/external/subtypes/standard.dm

@@ -258,6 +258,7 @@
 	damage_force = 3
 	throw_force = 7
 
+	var/hair_opacity = 255
 	var/can_intake_reagents = TRUE
 	var/eyes_over_markings = FALSE
 	var/eye_icon = "eyes_s"