move overmap_terrain definitions to json/overmap/overmap_terrain (#84516)

Author: Blueflowerss

data/json/mapgen/reverberations/cabins.json

@@ -1,27 +1,4 @@
 [
-  {
-    "type": "overmap_terrain",
-    "id": [ "cabin_3_reverberation" ],
-    "name": "cabin",
-    "sym": "C",
-    "color": "i_green",
-    "vision_levels": "isolated_building",
-    "travel_cost_type": "structure",
-    "see_cost": "high",
-    "mondensity": 0,
-    "flags": [ "RISK_LOW", "SOURCE_SAFETY", "SOURCE_FOOD" ]
-  },
-  {
-    "type": "overmap_terrain",
-    "id": [ "cabin_roof_3_reverberation" ],
-    "vision_levels": "roof_or_air",
-    "travel_cost_type": "roof",
-    "name": "cabin roof",
-    "sym": "C",
-    "color": "i_green",
-    "see_cost": "none",
-    "extras": "build"
-  },
   {
     "om_terrain": "cabin_3_reverberation",
     "type": "mapgen",

data/json/overmap/overmap_terrain/overmap_terrain_reverberations.json

@@ -0,0 +1,25 @@
+[
+  {
+    "type": "overmap_terrain",
+    "id": [ "cabin_3_reverberation" ],
+    "name": "cabin",
+    "sym": "C",
+    "color": "i_green",
+    "vision_levels": "isolated_building",
+    "travel_cost_type": "structure",
+    "see_cost": "high",
+    "mondensity": 0,
+    "flags": [ "RISK_LOW", "SOURCE_SAFETY", "SOURCE_FOOD" ]
+  },
+  {
+    "type": "overmap_terrain",
+    "id": [ "cabin_roof_3_reverberation" ],
+    "vision_levels": "roof_or_air",
+    "travel_cost_type": "roof",
+    "name": "cabin roof",
+    "sym": "C",
+    "color": "i_green",
+    "see_cost": "none",
+    "extras": "build"
+  }
+]