Fix types to support void async transformers

Related to #155.

Author: wooorm

index.d.ts

@@ -432,7 +432,7 @@ type Transformer = (
   node: Node,
   file: VFile,
   next: TransformCallback
-) => Promise<Node | undefined> | Node | Error | undefined | void
+) => Promise<Node | undefined | void> | Node | Error | undefined | void
 
 /**
  * Callback you must call when a transformer is done.

test/async-function.js

@@ -10,6 +10,14 @@ test('async function transformer () {}', (t) => {
   t.plan(5)
 
   unified()
+    .use(() => {
+      return async function () {}
+    })
+    .use(() => {
+      return async function () {
+        return undefined
+      }
+    })
     .use(() => {
       return async function (tree, file) {
         t.equal(tree, givenNode, 'passes correct tree to an async function')