Enable custom rules to use the micromark parser, export micromark helpers for reuse.

helpers/.npmignore

@@ -0,0 +1 @@
+test.js

helpers/package.json

@@ -3,7 +3,10 @@
   "version": "0.26.0",
   "description": "A collection of markdownlint helper functions for custom rules",
   "main": "./helpers.js",
-  "exports": "./helpers.js",
+  "exports": {
+    ".": "./helpers.js",
+    "./micromark": "./micromark-helpers.cjs"
+  },
   "author": "David Anson (https://dlaa.me/)",
   "license": "MIT",
   "homepage": "https://github.com/DavidAnson/markdownlint",

helpers/test.cjs

@@ -0,0 +1,28 @@
+// @ts-check
+
+"use strict";
+
+// eslint-disable-next-line n/no-extraneous-require
+const test = require("ava").default;
+const { "exports": packageExports, name } = require("../helpers/package.json");
+
+const exportMappings = new Map([
+  [ ".", "../helpers/helpers.js" ],
+  [ "./micromark", "../helpers/micromark-helpers.cjs" ]
+]);
+
+test("exportMappings", (t) => {
+  t.deepEqual(
+    Object.keys(packageExports),
+    [ ...exportMappings.keys() ]
+  );
+});
+
+for (const [ exportName, exportPath ] of exportMappings) {
+  test(exportName, (t) => {
+    t.is(
+      require(exportName.replace(/^\./u, name)),
+      require(exportPath)
+    );
+  });
+}