[w3c/webcomponents] has="mixin this, mixin that" (#663)

`has="mixin"` Idea for managing mixins without conflicting with existing classes or `is=` attribute. A followup to #662 #509 re `is=`.

Goal: address practical need for mixins with existing and custom elements, hook into element lifecycle, not conflict with various libraries, expectations in the wild.

```
...Sorter...
customElements.mixin('sortable', Sorter);


<any has="sortable"></any>
<any has="sortable, filterable, whatever"></any>
```

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