Port Tendrils feature generation (#46)

Add the writable symbol, adapt Legacy's branching solid cave-node geometry across all terrain types, and test authored and random Age selection.

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

@@ -0,0 +1,8 @@
+{
+  "schema_version": 2,
+  "name": "symbol.rivencraft.feature.tendrils",
+  "description": "symbol.rivencraft.feature.tendrils.description",
+  "category": "rivencraft:feature",
+  "semantics": { "role": "feature", "value": "rivencraft:tendrils" },
+  "poem": "Terrain Thread Structure Winding"
+}