Support incremental definitions of token renderers and renderer prototypes in the \markdownSetup command

[Witiko]

This PR adds support to incremental definitions of token renderers and renderer prototypes in the \markdownSetup plain TeX command using the following syntax:

\markdownSetup {
  renderers = {  % or rendererPrototypes
    headingOne += { code },  % Appends `code` to the top-level heading.
    % ...
  }
}

(Originally drafted in #232 (comment).)

This PR continues #232.

Tasks

• Implement prepending/appending.
• Unimplement prepending and change the syntax of appending from $= to +=.
  It's unclear that prepending (^=) has useful uses.
  Furthermore, characters ^ and $ in keys are difficult to parse due to their catcodes.
• Change \g_@@_appending_bool to \g_@@_appending_renderer_bool and renderer_prototype_bool.
• Reimplement appending without etoolbox, so that TeX formats other than LaTeX can easily be supported.
• Redefine default headerAttributeContextBegin renderer prototype for LaTeX using appending, as discussed in #232.
• Document appending in renderers and renderer prototypes.
• Ensure no TODOs remain in the Files changed tab.
• Review the text of the technical documentation.
• Update CHANGES.md.