Re: [w3ctag/design-reviews] HTMLScriptElement.supports(type) method (#674)

This API is proposing a nicer and synchronous way to feature detect.

For `module`, we can use the `nomodule` attribute to detect the `module` type support as [this example](https://html.spec.whatwg.org/multipage/scripting.html#script-nomodule-example) shows.

> https://html.spec.whatwg.org/multipage/scripting.html#script-nomodule-example
> ```html
> <script type="module" src="app.mjs"></script>
> <script nomodule defer src="classic-app-bundle.js"></script>
> ```

(Note: Safari 10 did support `module`, but didn't support `nomodule`. So we need some hack like [this solution](https://gist.github.com/samthor/64b114e4a4f539915a95b91ffd340acc) not to load `<script nomodule>`.)

For `importmap`, there is no synchronous way to detect  the `importmap` type support. As described in [the issue comment](https://github.com/WICG/import-maps/issues/171#issuecomment-521299519), we need to fetch an useless HTTP request.

> https://github.com/WICG/import-maps/issues/171#issuecomment-521299519
> ```html
> <script type="importmap">
> {
>   "imports": {
>     "vue": "/vendor/vue.js"
>   }
> }
> </script>
> // ...
> <script>
> import ("vue").catch(() => {
>   // We'll get here if we're in a browser that doesn't support import maps,
>   // because import "vue" will fail (with no evaluation performed). In that case
>   // the <script type=module> was also a no-op (mostly).
> 
>   const s = document.createElement('script');
>   s.src = "./bundle.js";
>   document.body.append(s);
> });
> </script>


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