Add tests for exposed identifiers

Author: wooorm

test.js

@@ -9,8 +9,13 @@ import {mdxjs} from 'micromark-extension-mdxjs'
 import {mdxFromMarkdown} from 'mdast-util-mdx'
 import {toHast} from 'mdast-util-to-hast'
 import {excerpt} from './index.js'
+import * as mod from './index.js'
 
 test('excerpt', () => {
+  const api = Object.keys(mod)
+  assert.ok(api.includes('excerpt'), 'should expose `excerpt`')
+  assert.equal(api.length, 1, 'should expose the public api')
+
   assert.deepEqual(
     excerpt(
       h('div', [