Re: [webcomponents] Support Custom Pseudo-elements (#300)

We would prefer using `part` syntax as follows so that future pseudo element names we introduce in CSS wouldn't interfere with author defined parts components expose:

```html
<date-range-selector>
  <!-- #shadow-root -->
    <div id="container">
      <input part="start-date" id="start-date" type="date">
      <input part="end-date" id="end-date" type="date">
    </div>
  <!-- /shadow-root -->
</date-range-selector>
```

```css
date-range-selector:part(start-date),
date-range-selector:part(end-date) {
  /* normal styles */
}
```

@hober @othermaciej

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https://github.com/w3c/webcomponents/issues/300#issuecomment-136716945