Add support for universal (cross-format) themes

[Witiko]

Since version 2.10.0 of the Markdown package, we have supported LaTeX themes and snippets. We should move the concepts of themes and snippets from the LaTeX layer to the plain TeX + expl3 layer (see a recent layer diagram of the Markdown package). On-disk, a theme would correspond to different files for different TeX formats. For example, the theme witiko/markdown/example would load the following files in different formats:

• markdownthemewitiko_markdown_example.tex in plain TeX
• markdownthemewitiko_markdown_example.sty in LaTeX
• t-markdownthemewitiko_markdown_example.tex in ConTeXt
• markdownthemewitiko_markdown_example.opm in OpTeX (blocked by Support OpTeX #215)

This architecture enables the creation of universal (aka fat) Markdown themes that work across different TeX formats. Formats without their format-specific theme would fall back to the plain TeX theme and issue a warning.

Besides the utility of universal themes for third parties, who can use Markdown as a unifying framework on top of different TeX formats, universal themes also present opportunities for simplifying the architecture of the Markdown package itself: We can neatly package all the default definitions of renderer prototypes as a single universal theme witiko/markdown/defaults, which could become its own independent TeX package in the future similar to lt3luabridge. We can also simplify the architecture of unit tests by merging files markdownthemewitiko_markdown_test.sty and plain-setup.tex into a universal theme witiko/markdown/test.

This issue is currently blocked by #275.