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

Yeah, if we do add custom pseudo-elements, we need to do it namespaced, like `::part`.

That said, the CSSWG provisionally accepted [my @apply rule draft](https://tabatkins.github.io/specs/css-apply-rule/), which makes pseudo-elements unnecessary for WC.

That is, CSS variables handle *most* of the styling needs of WC already.  They only fall down when you want to offer the ability to *arbitrarily* style an element; there are too many properties to make it reasonable to offer enough variables for styling.  Pseudo-elements expose an element for arbitrary styling, so that's useful.  The @apply rule *also* does that, tho, and via the existing mechanism of CSS custom properties, so there's probably no need to add custom pseudo-elements.

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