[docx reader]: checkbox field is not converted into other formats

[jooyoungseo]

In pandoc v2.18, checkbox field contained in docx file is not converted into other formats, such as markdown and html.

I am attaching a test file:

checkbox_field.docx

pandoc checkbox_field.docx -o output.md

[cijcij]

in case from other formats to docx, the checkbox also not handled correctly……

see: #5198