Override Leaf toString method

lib/src/models/documents/attribute.dart

@@ -12,6 +12,7 @@ enum AttributeScope {
 class Attribute<T> {
   Attribute(this.key, this.scope, this.value);
 
+  /// Unique key of this attribute.
   final String key;
   final AttributeScope scope;
   final T value;

lib/src/models/documents/nodes/leaf.dart

@@ -101,6 +101,13 @@ abstract class Leaf extends Node {
     }
   }
 
+  @override
+  String toString() {
+    final keys = style.keys.toList(growable: false)..sort();
+    final styleKeys = keys.join();
+    return '⟨$value⟩$styleKeys';
+  }
+
   /// Adjust this text node by merging it with adjacent nodes if they share
   /// the same style.
   @override