commonjs -> ecmascript modules

[brandonros]

similar to https://github.com/tim-kos/node-retry/pull/87

paves the way for

<script type="importmap">
      {
        "imports": {
          "retry": "https://cdn.jsdelivr.net/gh/brandonros/node-retry@mjs/index.mjs",
          "async-retry": "https://cdn.jsdelivr.net/gh/brandonros/async-retry@patch-1/lib/index.js"
        }
      }
    </script>
    <script type="module">
      import retry from 'async-retry'