Gitenberg: support gfm for turndown

[bangank36]

Summary

• turndown is a library that is used to convert HTML to markdown, but the default config did not support Gutenberg blocks like table...
• Adding the gfm (github flavored markdown) support using turndown-plugin-gfm
  • Please mention that the plugin is not maintained mixmark-io/turndown-plugin-gfm#28
  • https://github.com/laurent22/joplin/tree/dev/packages/turndown-plugin-gfm
  • All those plugins is intended for node env, so I made a dirty es module from there
• Current state

Editor	Output

Implementation

• Import turndown-plugin-gfm

[bangank36]

Closed as merged,

• How about tasks lists? We may need to create a new list block on Gutenberg to comply with the task list markup?