css: add matcher test w/ libdom

src/css/libdom.zig

@@ -13,10 +13,14 @@ pub const Node = struct {
         return null;
     }
 
-    pub fn nextSibling(n: Node) ?Node {
+    pub fn nextSibling(n: Node) !?Node {
         const c = try parser.nodeNextSibling(n.node);
         if (c) |cc| return .{ .node = cc };
 
         return null;
     }
+
+    pub fn tag(n: Node) ![]const u8 {
+        return try parser.nodeName(n.node);
+    }
 };