Add multirow table header support

[bai-yi-bai]

As mentioned in #users > Table header @ 💬, support for https://github.com/asciidoctor/asciidoctor-extensions-lab/blob/main/lib/multirow-table-header-tree-processor.rb requires a one line addition to the convert_table function to include the rowspan information.

[mojavelinux]

This needs to first be accepted in Asciidoctor core before it can be accepted here. And I think this is also a spec issue, so the feature needs to be at least mentioned in the AsciiDoc Language project.