This adds support for <script type="module"> for loading JavaScript modules, as well as all the infrastructure necessary for resolving, fetching, parsing, and evaluating module graphs rooted at such <script type="module">s. This was spurred on by a request over in whatwg/loader#83 (comment). This should take care of whatwg/loader#83, whatwg/loader#84, and whatwg/loader#82. There is not much overla