Separate default token renderer prototype definitions to a universal theme witiko/markdown/defaults

[Witiko]

In #373, we added support for universal themes and we moved the TeX side of our testing framework to a universal theme witiko/markdown/test. However, we missed the opportunity to extract the default token renderer prototype definitions for plain TeX, LaTeX, ConTeXt into a universal theme witiko/markdown/defaults.

Tasks

• Add new tags for the plain TeX, LaTeX, ConTeXt, and OpTeX theme witiko/markdown/defaults to markdown.ins.
• Document and implement the theme in subsections Themes of sections Interface and Implementation.
• Update Makefile, Dockerfile, and markdown.dtx with instructions for packaging and installing the files of the theme.