Add expl3-style interface for LaTeX

[Witiko]

The current LaTeX interface for setting renderers and renderer prototypes uses camel-casing, which seems out-of-place in expl3 code. Furthermore, the current interface does not provide any cues for the arity of renderers and renderer prototypes, which makes it more difficult to read and write code without keeping the documentation of Markdown handy.

Therefore, it seems useful to have the option to write \__markdown_setup:n { renderers = { link:nnnn = { ... } } } instead of \markdownSetup { renderers = { link = { ... } } }. This addition can be done automatically due to the reflection capabilities of Markdown added in #119. In the future, other types of parameter passing (N) and expansion (x) could also be supported if there is a compelling use case.

Alternatively, we could have mnemonics for different types of parameters such as link:lUut, where l stands for label, U and u stand for escaped and raw URL, and t stands for title. This would make it possible for the programmer to reorder the parameters and omit those that they don't need:

\__markdown_setup:n
    {
        renderers = {
            link:t = { Here~is~the~title~of~the~link:~#1. }
        }
    }

[Witiko]

@PetrSojka I believe this has been your suggestion in one of my previous talks about Markdown.