In DOCX, like HTML, a table cell can itself contain a table. This is not
uncommon and is typically used for formatting purposes.
When a DOCX table is nested, create nested HTML tables to reflect that
structure and create a plain-text table with captures all the text in
nested tables, formatting it as a reasonable facsimile of a table.
This implements the solution described and spiked in PR #1952.
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Co-authored-by: Bruno Bornsztein <bruno.bornsztein@gmail.com>
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