[w3c/webcomponents] Mechanism for setting the tabindex focus flag without sprouting tabindex attribute? (#762)

[Certain HTML elements](https://html.spec.whatwg.org/multipage/interaction.html#the-tabindex-attribute:the-a-element) are focusable without having an explicit `tabindex` attribute:
* `a` elements that have an `href` attribute
* `link` elements that have an `href` attribute
* `button` elements
* `input` elements whose type attribute are not in the Hidden state
* `select` elements
* `textarea` elements
* `summary` elements that are the first summary element child of a details element

It would be nice if custom elements could also opt in to being focusable without having to sprout a `tabindex` attribute - such that if page authors removed the `tabindex` attribute, the element was still focusable.

It seems like this would be something that might fit in with whatever new API comes out of #758.

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