Added Markdown export, CommenType backtick "```"

[Lucas Zampieri]

When pasting an ASCII diagram to a platform that uses Markdown, such as Github, Gitlab, Notion. It Is needed to involve the diagram by a code block "```".

Add an option to export the diagram wrapped by the code block syntax.

Probably something like:
/client/export.tsx

  <MenuItem value={"code_block"}>
    Standard multi-line <CommentTypeChip label="``` ```" />
  </MenuItem>

[Lewis Hemens]

This is now in!