Bump up remark dependencies and support only ESM (#6)

[`remark@14`](https://github.com/remarkjs/remark/releases/tag/14.0.0) has been supported only ESM.
So, this package needs to become ESM to bump up `remark` dependencies.

Here are the installation commands:

```shell
npm install \
  remark-cli@latest \
  remark-frontmatter@latest \
  remark-preset-lint-recommended@latest \
  remark-preset-prettier@latest \
  remark-validate-links@latest

npm install 'github:stylelint/eslint-config-stylelint#esm'
```

Note that this change needs the ESLint configuration for ESM.

.remarkrc.js

@@ -1,10 +1,8 @@
-'use strict';
+import frontmatter from 'remark-frontmatter';
+import presetLintRecommended from 'remark-preset-lint-recommended';
+import validateLinks from 'remark-validate-links';
+import presetPrettier from 'remark-preset-prettier';
 
-module.exports = {
-	plugins: [
-		require('remark-frontmatter'),
-		require('remark-preset-lint-recommended'),
-		require('remark-validate-links'),
-		require('remark-preset-prettier'),
-	],
+export default {
+	plugins: [frontmatter, presetLintRecommended, validateLinks, presetPrettier],
 };

index.js

@@ -1,3 +1 @@
-'use strict';
-
-module.exports = require('./.remarkrc.js');
+export { default } from './.remarkrc.js';