`commonmark-pandoc`: calculate relative cell widths for pipe tables

Pandoc's Markdown parser sets relative widths for each column in a pipe table when the table contains long rows, which allows LaTeX to wrap cells to avoid overflowing the page (implemented in https://github.com/jgm/pandoc/commit/eb8aee477db045a7449bc752975528263964b8ce).

`commonmark-pandoc` doesn't implement this functionality and always sets the column width to default, resulting in poor PDF rendering:
https://github.com/jgm/commonmark-hs/blob/6ec393d5c7950b5edd410d448a00344277c971ad/commonmark-pandoc/src/Commonmark/Pandoc.hs#L139-L140

It'd be great if this feature could be implemented in `commonmark-pandoc`, but I don't have the familiarity with Haskell or the codebase to do it myself.