Asciidoc markdown block

Block extension to filter text like:

    [markdown]
    --
    Filter some text with [Markdown](https://commonmark.org/help/) syntax.
    --

This is not implemented with a full Markdown parser.
See asciidoctor/kramdown-asciidoc#7
with a link to the "naive series of regexes" used as starting point.

(And note that we use Open structural context
https://docs.asciidoctor.org/asciidoc/latest/blocks/delimited/#summary-of-structural-containers
with `--` delimiters, and headings don’t work inside these.)

This feature is intended for handling OpenAPI specs, which can contain Markdown,
however openapi-generator has poor Asciidoc handling
OpenAPITools/openapi-generator#11396
so instead we add the block delimiters in the template, and let the block filter handle it.
Create a block with potentially multiple lines (sub-paragraphs)

* Add extensions doc page

antora-playbook.yml

@@ -168,6 +168,7 @@ asciidoc:
   - ./lib/multirow-table-head-tree-processor.js
   - ./lib/swagger-ui-block-macro.js
   - ./lib/tabs-block.js
+  - ./lib/markdown-block.js
   - asciidoctor-kroki
 ui:
   bundle:

home/modules/contribute/pages/extensions.adoc

@@ -0,0 +1,36 @@
+= Site Extensions
+
+Test and document your Asciidoctor.js or Antora extensions here.
+
+== Markdown Block
+
+[source,asciidoc]
+----
+[markdown]
+--
+Filter some text with [Markdown](https://commonmark.org/help/) syntax.
+--
+----
+
+Results in:
+
+[markdown]
+--
+Filter some text with [Markdown](https://commonmark.org/help/) syntax.
+--
+
+
+[NOTE]
+--
+This is not implemented with a full Markdown parser.
+See link:https://github.com/asciidoctor/kramdown-asciidoc/issues/7[issue]
+with a link to the "naive series of regexes" used as starting point.
+
+(And note that we use Open 
+link:https://docs.asciidoctor.org/asciidoc/latest/blocks/delimited/#summary-of-structural-containers[structural context], 
+with `--` delimiters, and headings don't work inside these.)
+
+This feature is intended for handling OpenAPI specs, which can contain Markdown,
+however openapi-generator has link:https://github.com/OpenAPITools/openapi-generator/issues/11396[poor Asciidoc handling],
+so instead we add the block delimiters in the template, and let the block filter handle it.
+--

lib/markdown-block.js

@@ -0,0 +1,42 @@
+// from https://github.com/dohliam/markdoctor/blob/master/js/mdtoadoc.js
+function convMdAdoc(txt) {
+  // horizontal rules
+  txt = txt.replace(/^\-{3,}$/gm, "'''").replace(/^[\*_]{3,}/gm, "'''");
+
+  // general text formatting
+  txt = txt.replace(/^\*([^\s\*][^\*]+)\*/gm, "_$1_").replace(/\s\*([^\*\s]+)\*/g, " _$1_").replace(/\*\*_/g, "*_").replace(/_\*\*/g, "_*").replace(/^\*\*([^\*]+)\*\*/gm, "*$1*").replace(/\s\*\*([^\*]+)\*\*/g, " *$1*");
+
+  // headings
+  txt = txt.replace(/^#*/gm, (x) => '='.repeat(x.length))
+
+  // images
+  txt = txt.replace(/\[\!\[([^\]]*)\]\(([^\)]+)\)\]\(([^\)]+)\)/g, "image::$2[$1, link=\"$3\"]").replace(/\!\[([^\]]*)\]\(([^\)]+)\)/g, "image::$2[$1]");
+
+  // links
+  txt = txt.replace(/\[([^\]]+)\]\(([a-z\.\/]+[^:\)]+)\)/g, "link:$2[$1]").replace(/\[([^\]]+)\]\(([a-z]+:\/\/[^\)]+)\)/g, "$2[$1]");
+
+  // unordered lists
+  txt = txt.replace(/^\s{2}[\*\-]\s/gm, "** ").replace(/^\s{4}[\*\-]\s/gm, "*** ").replace(/^\s{6}[\*\-]\s/gm, "**** ").replace(/^\s{8}[\*\-]\s/gm, "***** ");
+
+  // ordered lists
+  txt = txt.replace(/^\d+\.\s/gm, ". ").replace(/^\s{2}\d+\.\s/gm, ".. ").replace(/^\s{4}\d+\.\s/gm, "... ").replace(/^\s{6}\d+\.\s/gm, ".... ").replace(/^\s{8}\d+\.\s/gm, "..... ");
+
+  // tables
+  txt = txt.replace(/^\|*\s*(.*?)\s+\|\s+(.*?)\n\|*\s*\-+.*\n/gm, "[options=\"header\"]\n|===\n| $1 | $2\n").replace(/^\|*\s*(.*?\s+\|\s+.*?)\n\n/gm, "| $1\n|===\n\n").replace(/^([^\|]+\s+\|\s+.*$)/gm, "| $1");
+
+  return txt
+}
+
+module.exports = function (registry) {
+  registry.block(function () {
+    var self = this
+    self.named('markdown')
+    self.onContext('open')
+    self.process(function (parent, reader) {
+
+      var lines = reader.getLines().map(function (l) { return convMdAdoc(l) })
+      const source = lines.join('\n')
+      return self.createOpenBlock(parent, source)
+    })
+  })
+}