[webcomponents] Restrict the types of nodes that could be assigned to a slot (#351)

It seems that we only want Text and Element nodes to be assigned to a slot.  There is currently no mechanism to assign non-element nodes to a slot other than via the default slot so this is only relevant to default slots.  Since only Text and Element nodes are visible, we can probably not even expose `assignedSlot` on non-Text CharacterData nodes.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/351