Add support of direct mappings of markdown elements to commands

[Witiko]

The Lua parser of the Markdown package produces renderers, which are then typeset using the expansion processor of TeX. This is a simple design that works for most use cases and makes the output of the Lua parser predictable and amenable to static analysis by third-party tools. However, it also fails to account for commands such as \verb, \end{verbatim}, and \end{frame} that are not processed by TeX's expansion processor and must appear at the top level of the document to be correctly processed.

To support commands that cannot be produced from renderers using expansion, the Markdown package may need a direct mapping mechanism, which would allow the Lua parser to directly produce the commands and their parameters instead of renderers. The direct mapping mechanism could be fully configured from plain TeX as well as from other TeX formats. The direct mapping mechanism will likely make the design of the markdown package more complicated and the output of the Lua parser less useful for third-party tools.

The idea of the direct mapping of markdown elements has been presented at TUG 2021.