Add tests to verify .markdownlint.cjs/mjs and .markdownlint-cli2.cjs/…

…mjs can return the configuration/options object from a Promise, update documentation.

README.md

@@ -352,7 +352,7 @@ of the rules within.
 
 - The format of this file is a [CommonJS module][commonjs-module] (`.cjs`) or
   [ECMAScript module][ecmascript-module] (`.mjs`) that exports the object
-  described above for `.markdownlint-cli2.jsonc`.
+  described above for `.markdownlint-cli2.jsonc` (directly or from a `Promise`).
 - Instead of passing a `String` to identify the module name/path to load for
   `customRules`, `markdownItPlugins`, and `outputFormatters`, the corresponding
   `Object` or `Function` can be provided directly.
@@ -392,7 +392,8 @@ of the rules within.
 
 - The format of this file is a [CommonJS module][commonjs-module] (`.cjs`) or
   [ECMAScript module][ecmascript-module] (`.mjs`) that exports the
-  [`markdownlint` `config` object][markdownlint-config].
+  [`markdownlint` `config` object][markdownlint-config] (directly or from a
+  `Promise`).
 - Other details are the same as for `jsonc`/`json` files described above.
 - For example: [`.markdownlint.cjs`][markdownlint-cjs] or
   [`.markdownlint.mjs`][markdownlint-mjs]

test/markdownlint-cjs/dir/subdir/.markdownlint.cjs

@@ -2,6 +2,6 @@
 
 "use strict";
 
-module.exports = {
+module.exports = Promise.resolve({
   "first-line-heading": false
-};
+});

test/markdownlint-cli2-cjs/dir/subdir/.markdownlint-cli2.cjs

@@ -2,8 +2,8 @@
 
 "use strict";
 
-module.exports = {
+module.exports = Promise.resolve({
   "config": {
     "first-line-heading": false
   }
-};
+});

test/markdownlint-cli2-mjs/dir/subdir/.markdownlint-cli2.cjs → test/markdownlint-cli2-mjs/dir/subdir/.markdownlint-cli2.mjs

@@ -1,9 +1,9 @@
 // @ts-check
 
-"use strict";
-
-module.exports = {
+const options = Promise.resolve({
   "config": {
     "first-line-heading": false
   }
-};
+});
+
+export default options;

test/markdownlint-mjs/dir/subdir/.markdownlint.mjs

@@ -1,7 +1,7 @@
 // @ts-check
 
-const config = {
+const config = Promise.resolve({
   "first-line-heading": false
-};
+});
 
 export default config;