preserve more row/column information in tikz matrices

[xworld21]

Pie-in-the-sky request: preserve enough data about TikZ matrices so that the tabular structure can be reconstructed in postprocessing or even just in Javascript. (The goal is to attach appropriate ARIA roles, so that screen readers can navigate TikZ matrices as if they were tables.)

Some of the data is already present, as pgfsys-latexml tags cells with special classes (like ltx_tikzmatrix_row). Unfortunately, empty cells are removed from the output so it's not possible to programmatically determine which cell belongs to which column.