Update Text Node clone method to clone value as well

Author: singerdmx

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

@@ -211,7 +211,7 @@ class Text extends Leaf {
         super.val(text);
 
   @override
-  Node newInstance() => Text();
+  Node newInstance() => Text(value);
 
   @override
   String get value => _value as String;