feat: support parsing the nested list when exporting the document to markdown format

[LucasXu0]

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Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d1027c4) 80.25% compared to head (0484c38) 80.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
+ Coverage   80.25%   80.30%   +0.04%     
==========================================
  Files         282      282              
  Lines       12056    12061       +5     
==========================================
+ Hits         9675     9685      +10     
+ Misses       2381     2376       -5     

Files	Coverage Δ
...ns/markdown/encoder/document_markdown_encoder.dart	100.00% <100.00%> (ø)
...down/encoder/parser/bulleted_list_node_parser.dart	100.00% <100.00%> (+10.00%)	⬆️
...arkdown/encoder/parser/code_block_node_parser.dart	90.90% <ø> (ø)
...s/markdown/encoder/parser/heading_node_parser.dart	100.00% <100.00%> (+8.33%)	⬆️
...ins/markdown/encoder/parser/image_node_parser.dart	100.00% <ø> (ø)
...c/plugins/markdown/encoder/parser/node_parser.dart	100.00% <ø> (ø)
...down/encoder/parser/numbered_list_node_parser.dart	100.00% <100.00%> (+10.00%)	⬆️
...ins/markdown/encoder/parser/table_node_parser.dart	100.00% <ø> (ø)
...gins/markdown/encoder/parser/text_node_parser.dart	100.00% <100.00%> (+12.50%)	⬆️
...markdown/encoder/parser/todo_list_node_parser.dart	100.00% <100.00%> (+8.33%)	⬆️
... and 1 more

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