Modernize LaTeX code using xparse and expl3

[Witiko]

At the moment, the LaTeX implementation makes heavy use of the keyval and xstring third-party packages, and provides unintuitive interface (\begin{markdown} and \begin{markdown*}{options} instead of \begin{markdown} and \begin{markdown}[options]).

Instead of using third-party packages, we should modernize the implementation to use expl3 for key-value methods instead of keyval and for string comparisons instead of xstring, and use xparse to provide more intuitive interface (\begin{markdown} and \begin{markdown}[options] with \begin{markdown*}{options} as a deprecated syntax).

Tasks

• Use expl3 to replace calls to xstring. (See Replace calls to xstring with expl3 functions #153.)
• Use expl3 to replace calls to keyval. (See Replace keyval with l3keys #157. Check that this closes Make the \markdownSetup and \markdownSetupSnippet commands accept \par tokens #130.)
• Use xparse to add an optional parameter to the markdown environment and deprecate the markdown* environment.
  Use the new form in examples/latex.tex. (See Use xparse to add an optional parameter to the markdown environment and deprecate the markdown* environment #158.)

References

• The expl3 package and LaTeX3 programming
• Functions and expl3
• Programming key–value in expl3
• The xparse package: Document command parser
• The LaTeX3 Interfaces
• xstring: String manipulation for (La)TeX
• keyval: Process key=value schemes

See also other issues with the expl3 label.

[Witiko]

This issue is blocked by #124.

[Witiko]

Closing this issue also closes #96.

[Witiko]

This issue is no longer blocked by #124 (closed).