[webcomponents] attachShadow should fail on a slot element (#327)

I don't think it makes sense to support `attachShadow` on slot elements since they don't even render by default due to its default style `display: contents` unless there is a compelling use case for it.

We should add it to the list of forbidden elements in https://w3c.github.io/webcomponents/spec/shadow/#widl-Element-attachShadow-ShadowRoot-ShadowRootInit-shadowRootInitDict.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/327