Add mod-aware Dense Ores age feature (#55)

src/main/resources/data/rivencraft/rivencraft/symbols/feature/dense_ores.json

@@ -0,0 +1,8 @@
+{
+  "schema_version": 2,
+  "name": "symbol.rivencraft.feature.dense_ores",
+  "description": "symbol.rivencraft.feature.dense_ores.description",
+  "category": "rivencraft:feature",
+  "semantics": { "role": "feature", "value": "rivencraft:dense_ores" },
+  "poem": "Nature Material Treasure Abundance"
+}

src/main/resources/data/rivencraft/tags/worldgen/placed_feature/dense_ores.json

@@ -0,0 +1,4 @@
+{
+  "replace": false,
+  "values": []
+}