loader: fix types

Closes GH-2386.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

packages/loader/index.cjs

@@ -7,6 +7,10 @@
 
 'use strict'
 
+// Note: we can’t export immediately, as TS generates broken types.
+// See: mdx-js/mdx#2386.
+module.exports = loader
+
 /**
  * Webpack loader
  *
@@ -19,7 +23,7 @@
  * @returns {undefined}
  *   Nothing.
  */
-module.exports = function (code) {
+function loader(code) {
   const callback = this.async()
   // Note that `import()` caches, so this should be fast enough.
   import('./lib/index.js').then((module) => {