Add World.tag to created Tag by type

src/world.zig

@@ -67,7 +67,16 @@ pub fn World(comptime ctx: anytype) type {
             return Entity(ctx).new(self, config, add);
         }
 
+        pub fn tag(self: *Self, comptime T: type) !Entity(ctx) {
+            if (@sizeOf(T) > 0) @compileError("'" ++ @typeName(T) ++ "' must be a zero-sized type");
+            const name = util.simpleTypeName(T);
+            const result = try self.entity(.{ .name = name, .symbol = name }, .{});
+            Lookup(ctx, T).id = result.raw;
+            return result;
+        }
+
         pub fn component(self: *Self, comptime T: type) !Entity(ctx) {
+            if (@sizeOf(T) == 0) @compileError("'" ++ @typeName(T) ++ "' must not be a zero-sized type");
             const name = util.simpleTypeName(T);
             const entity2 = try self.entity(.{ .name = name, .symbol = name, .use_low_id = true }, .{});