[whatwg/dom] Declarative Shadow DOM (#831)

I would like to re-open the topic of declarative Shadow DOM. This has been discussed in the past, [here on WHATWG](https://github.com/whatwg/dom/issues/510), in W3C [here](https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Declarative-Shadow-DOM.md) and [here](https://github.com/w3c/webcomponents/issues/71), and in [WICG](https://discourse.wicg.io/t/declarative-shadow-dom/1904/8). The last substantive public discussion was at the [Tokyo Web Components F2F](https://github.com/whatwg/dom/issues/510#issuecomment-370980398), where it was resolved **not** to proceed. I would like to revisit that decision.

I think declarative Shadow DOM is an important feature that is missing from the Web, and is something that we should try to implement. The primary motivating use case for declarative Shadow DOM is Server Side Rendering (SSR), which is practically difficult or impossible to use in combination with Shadow DOM. There are also other compelling use cases such enabling scoped styles without requiring Javascript. The rationale behind the prior decision not to proceed with this feature was largely a) implementation complexity and b) lack of developer need.

To address these points, and to explore the topic further, I've written up an explainer, here:

https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md


I believe this document captures most of the details of the motivation, required features, contentious points, and prior history. But I would love to hear your thoughts and feedback so that this proposal can evolve into something implementable and standardizable. I'm hoping we can use this thread as a discussion forum.

As a quick summary of the proposed syntax, this HTML:

```html
<host-element>
    <template shadowroot="open">
        <style>shadow styles</style>
        <h2>Shadow Content</h2>
        <slot></slot>
    </template>
    <h2>Light content</h2>
</host-element>
```

would be parsed into this DOM tree:

```html
<host-element>
  #shadow-root (open)
    <style>shadow styles</style>
    <h2>Shadow Content</h2>
    <slot>
        ↳ <h2> reveal
    </slot>
  <h2>Light content</h2>
</host-element>
```

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