Re: [whatwg/dom] Declarative Shadow DOM (#510)

shadow: none|open|closed, default is none
```html
<template shadow="open"></template>
<element shadow="open">
    <template></template>
</element>
```
```javascript
customElements.define('custom-element', class extends HTMLElement{}, {
    shadow: 'open'
});
// or
customElements.define('custom-element', class extends HTMLElement {
    static get shadow() { return 'open'; }
});
```

```html
<the-element>
    <shadow mode="open">
        <h1>Shadow Contents</h1>
        <slot></slot>
    </shadow>
    <h1>Contents</h1>
</the-element>
<!--parsed as-->
<the-element>
    #shadowroot (open)
        <h1>Shadow Contents</h1>
        <slot></slot>
    <h1>Contents</h1>
</the-element>
```

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