Implemented Entity tagged union for highlighted entity (similar to abstract class in some ways)

src/common/structures.zig

@@ -0,0 +1,7 @@
+const Road = @import("../infrastructure/road.zig").Road;
+const Node = @import("../infrastructure/node.zig").Node;
+
+pub const Entity = union(enum) {
+    node: *Node,
+    road: *Road,
+};