Implement Markdown definition lists (+ migration) (#20333)

Implements definition lists Markdown extension adopted in a few
implementations including:
* [Pandoc](
  https://pandoc.org/MANUAL.html#definition-lists)
* [kramdown](
  https://kramdown.gettalong.org/quickref.html#definition-lists)
* [PHP extra Markdown](
  https://michelf.ca/projects/php-markdown/extra/#def-list)

Also affected files have been migrated.
RST definition lists are turned off for Markdown: this solves the
problem of broken formatting mentioned in
https://github.com/nim-lang/Nim/pull/20292.

compiler/nimpaths.nim

@@ -9,7 +9,7 @@ specialpaths is simpler because it doesn't need variables to be relocatable at
 runtime (eg for use in testament)
 
 interpolation variables:
-  $nimr: such that `$nimr/lib/system.nim` exists (avoids confusion with $nim binary)
+: $nimr: such that `$nimr/lib/system.nim` exists (avoids confusion with $nim binary)
          in compiler, it's obtainable via getPrefixDir(); for other tools (eg koch),
         this could be getCurrentDir() or getAppFilename().parentDir.parentDir,
         depending on use case