Properly marks merge conflicts in map editor (#3974)

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## About The Pull Request
Fixes #3965

(re)Adds a merge conflict marker to be used by mapmerge2, additionally
adds an icon for it.


![image](https://github.com/user-attachments/assets/bcfc37e4-a50a-442b-96d0-d1c42419743c)

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## Why It's Good For The Game
Merge conflicts can be properly fixed in the map editor now
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benefit the game. If you can't justify it in words, it might not be
worth adding. -->

## Changelog

:cl:
fix: merge conflicts are now properly marked
/:cl:

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code/game/objects/merge_conflict_marker.dm

@@ -0,0 +1,6 @@
+/obj/merge_conflict_marker
+	name = "Merge Conflict Marker" // name & desc are set by merge script
+	desc = "Marks a merge conflict that needs manually resolved. If you're reading this, something has gone very wrong."
+	icon = 'icons/effects/mapping/mapping_helpers.dmi'
+	icon_state = "mergeconflict"
+	layer = ABOVE_ALL_MOB_LAYER

shiptest.dme

@@ -200,8 +200,8 @@
 #include "code\__HELPERS\game.dm"
 #include "code\__HELPERS\generators.dm"
 #include "code\__HELPERS\global_lists.dm"
-#include "code\__HELPERS\hearted.dm"
 #include "code\__HELPERS\heap.dm"
+#include "code\__HELPERS\hearted.dm"
 #include "code\__HELPERS\icon_smoothing.dm"
 #include "code\__HELPERS\icons.dm"
 #include "code\__HELPERS\level_traits.dm"
@@ -1075,6 +1075,7 @@
 #include "code\game\objects\buckling.dm"
 #include "code\game\objects\empulse.dm"
 #include "code\game\objects\items.dm"
+#include "code\game\objects\merge_conflict_marker.dm"
 #include "code\game\objects\obj_defense.dm"
 #include "code\game\objects\objs.dm"
 #include "code\game\objects\structures.dm"
@@ -3302,8 +3303,6 @@
 #include "code\modules\ruins\objects_and_mobs\ash_walker_den.dm"
 #include "code\modules\ruins\objects_and_mobs\necropolis_gate.dm"
 #include "code\modules\ruins\objects_and_mobs\sin_ruins.dm"
-#include "code\modules\ruins\rockplanet_ruin_code\mining_base.dm"
-#include "code\modules\ruins\sandplanet_ruin_code\cave_base.dm"
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazard_info.dm"
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazard_code\_hazard.dm"
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazard_code\_shutoff.dm"
@@ -3317,6 +3316,8 @@
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazards\generic_electrical.dm"
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazards\radioactive_barrels.dm"
 #include "code\modules\ruins\objects_and_mobs\generic_hazards\hazards\water_hazard.dm"
+#include "code\modules\ruins\rockplanet_ruin_code\mining_base.dm"
+#include "code\modules\ruins\sandplanet_ruin_code\cave_base.dm"
 #include "code\modules\ruins\spaceruin_code\bigderelict1.dm"
 #include "code\modules\ruins\spaceruin_code\TheDerelict.dm"
 #include "code\modules\ruins\wasteplanet_ruin_code\tradepost.dm"