Improve LaTeX template update

[cderv]

Three ideas:

1. Check if the diff of a new Pandoc releases is relevant semi-automatically for all templates.
2. Add test cases, i.e. check if existing templates yield the same, expected output with a new Pandoc release (this would spot changes in variables passed from Pandoc to the template etc.).
3. Modularize, like Quarto is doing. Basically, they seem to break down each new Pandoc LaTeX template into what they call partials (sub-templates) and compile the entire template by importing those.

See also the GitHub repository and instructions for new formats.

This allows replacing just e.g. the TeX code for tables. The basic mechanism is that Quarto uses the default "partial" to assemble the complete template, unless a format includes a modified partial.

The advantage of this is that the individual adjustments become much more independent from the evolution of the Pandoc template as a whole.

The downside is that you may need to tweak RStudio to implement the mechanism (but that sounds not too difficult).

Originally posted by @mfhepp in #410 (comment)

[cderv]

• Using Pandoc partials will require a requirement of Pandoc 2.9 which is from 2019-12-11
• $elseif$ is from Pandoc 2.8