Integrate Text.Pandoc.Builder with lua filter infrastructure

[wlupton]

Please see pandoc-ext/list-table#2 for context, and feel free to improve this issue's title.

The initial problem was that the list-table lua filter can generate a table with undefined cells, which causes the docx writer to fail. This raised the question of whether list-table should ensure that it defines all cells, and this in turn led to a suggestion that there could be a lua-exposed function that turns a malformed table into a well-formed one, with a note that the existing table builder in Text.Pandoc.Builder already does something like this.

I'm not sure whether the current thinking is that the Text.Pandoc.Builder table builder should be exposed to lua, or whether (more generally) ASTs returned from lua filters should be processed by Text.Pandoc.Builder (if that can be done efficiently).