fix links clickable offset

packages/zefyr/example/lib/src/images.dart

   @override
   void onLinkTap(ZefyrLinkEntity value) {
     // return
-    print('The word touched is 123123');
+    print(value.text);
   }
 }
 

packages/zefyr/lib/src/widgets/selection.dart

       if (_scope.controller.getStyleForSelection(selection).contains(NotusAttribute.link)) {
         // 当前点击的是link
         var position = paragraph.getRenderBoxPositionForOffset(localPoint);
-        // 此处加1是点击text尾，不算点中
+        // 第一个字符和最后一个字符将不触发onTap
         final TextSpan span = paragraph.text.getSpanForPosition(TextPosition(
-          offset: position.offset + 1,
+          offset: position.offset == 0 ? position.offset - 1 : position.offset + 1,
           affinity: position.affinity,
         ));
         final recognizer = (span?.recognizer as TapGestureRecognizer);