Support for SummaryTables.jl

[matthijscox-asml]

We recently found out that WriteDocx.jl exists and uses a special package called SummaryTables.jl to define complex tables with merged cells. This would be interesting to add to PPTX.jl, since we have similar table functionality.

I even wonder if the .docx XML schema for tables is equivalent to the .pptx tables schema. In which case we could re-use code from WriteDocx.jl.

SummaryTables.jl does not yet seem to have color support, so we cannot close issue 41, unless we find a way to add color support to SummaryTables.jl directly.

[matthijscox-asml]

Okay I thought SummaryTables.jl was a kind of table interface package, but it seems to be a concrete table implementation with conversions to all underlying table structures (LaTeX, typst, .docx).
So basically we would first need a full .pptx table structure in PPTX.jl (currently it's too simple), and then we could convert SummaryTables.jl to our table structure.

I briefly checked whether .docx and .pptx tables have the same XML schema by creating the same table in both documents and viewing the generated XML. There are some commonalities, but also lots of differences, so re-using WriteDocx.jl code seems out of the question.