Fix md image export

[rileyhawk1417]

When exporting the document to markdown the image would show up as ![](). The image url/src didn't get passed.

In the image_node_parser.dart its looking for the attribute image_src which doesnt exist.

  @override
  String transform(Node node, DocumentMarkdownEncoder? encoder) {
    return '![](${node.attributes['image_src']})';
  }

However when checking the attributes url is passed and not image_src

[LucasXu0]

@rileyhawk1417 This test failed.

    test('parser image node', () {
      final node = Node(
        type: 'image',
        attributes: {
          'image_src': 'https://appflowy.io',
        },
      );

      final result = const ImageNodeParser().transform(node, null);
      expect(result, '![](https://appflowy.io)');
    });

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (66762ef) 75.57% compared to head (4139362) 75.57%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #713   +/-   ##
=======================================
  Coverage   75.57%   75.57%           
=======================================
  Files         297      297           
  Lines       13735    13735           
=======================================
  Hits        10380    10380           
  Misses       3355     3355           

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