[w3c/webcomponents] Repurpose the is="" attribute (or make a new attribute with a new name) (#662)

Currently, `is=""` is spec'd to define ways in which a Custom Element can extend from a native element but without having to write the Custom Element's name inside the DOM because it would otherwise break ancient parsing rules and therefore cause unexpected breaking behavior which leads to confusion. See issue https://github.com/w3c/webcomponents/issues/509 detailing the problems with the currently-spec'd behavior.

For example, one of the great uses for `is=""` is that is solve problems with tables and other elements, where using custom element the normal way will not work:

```html
<table>
  <my-tr></my-tr>
</table>
```

Instead the current spec allows for

```html
<table>
  <tr is="my-tr"></tr>
</table>
```

so that the table can be parsed and rendered correctly, but this currently requires awkward and confusing inheritance design as mentioned in #509.

---

Here's a very simple proposal that would repurpose `is=""` (or an attribute with a new name): TODO: INSERT LINK HERE.

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