Add core.spawn_tree_on_vmanip (#15415)

This function works like `core.spawn_tree`, but spawns an L-system tree onto a VoxelManip object instead on the map.

src/mapgen/treegen.cpp

@@ -875,4 +875,15 @@ void make_pine_tree(MMVManip &vmanip, v3s16 p0, const NodeDefManager *ndef,
 	}
 }
 
+std::string error_to_string(error e)
+{
+	switch (e) {
+		case SUCCESS:
+			return "success";
+		case UNBALANCED_BRACKETS:
+			return "closing ']' has no matching opening bracket";
+	}
+	return "unknown error";
+}
+
 }; // namespace treegen

src/mapgen/treegen.h

@@ -68,4 +68,7 @@ namespace treegen {
 	treegen::error make_ltree(MMVManip &vmanip, v3s16 p0, const TreeDef &def);
 	// Helper to spawn it directly on map
 	treegen::error spawn_ltree(ServerMap *map, v3s16 p0, const TreeDef &def);
+
+	// Helper to get a string from the error message
+	std::string error_to_string(error e);
 }; // namespace treegen