Add Rule.parser property for custom rules to specify which Markdown parser output they use, honor it strictly, and add full type support.

lib/md023.js

@@ -12,6 +12,7 @@ module.exports = {
   "names": [ "MD023", "heading-start-left" ],
   "description": "Headings must start at the beginning of the line",
   "tags": [ "headings", "spaces" ],
+  "parser": "markdownit",
   "function": function MD023(params, onError) {
     filterTokens(params, "heading_open", function forToken(token) {
       const { lineNumber, line } = token;