Fixes forecon marksman being hit by bullets while invisible (#8427)

# About the pull request

Adds faction_marine to forecon preventing the marksman from being hit
while cloaked

# Explain why it's good for the game
fixes #7553 


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
fix: Fixes forecon spotter/marksman being hit by ally bullets while
inivsible
/:cl:

---------

Co-authored-by: InsaneRed <prodexter31@outlook.comm>
Co-authored-by: harryob <55142896+harryob@users.noreply.github.com>

code/modules/mob/mob_helpers.dm

@@ -486,6 +486,24 @@ GLOBAL_LIST_INIT(limb_types_by_name, list(
 /mob/proc/can_see_reagents()
 	return stat == DEAD || issynth(src) || HAS_TRAIT(src, TRAIT_REAGENT_SCANNER) //Dead guys and synths can always see reagents
 
+/// Returns TRUE if this mob is an ally of another, depending on the `faction` and `faction_group` variables, FALSE otherwise
+/mob/proc/is_ally_of(mob/potential_ally)
+	. = FALSE
+
+	if(faction == potential_ally.faction)
+		return TRUE
+
+	if(faction in potential_ally.faction_group)
+		return TRUE
+
+	if(potential_ally.faction in faction_group)
+		return TRUE
+
+	if(length(faction_group & potential_ally.faction_group))
+		return TRUE
+
+	return FALSE
+
 /**
  * Examine a mob
  *

code/modules/projectiles/projectile.dm

@@ -871,7 +871,7 @@
 			. -= mobility_aura * 5
 		var/mob/living/carbon/human/shooter_human = P.firer
 		if(istype(shooter_human))
-			if(shooter_human.faction == faction && !(ammo_flags & AMMO_ALWAYS_FF))
+			if(is_ally_of(shooter_human) && !(ammo_flags & AMMO_ALWAYS_FF))
 				. -= FF_hit_evade
 
 			if(ammo_flags & AMMO_MP)