[w3c/webcomponents] Provide a declarative way for customElement/template binding (#850)

There is currently no way to use the `template` element without Javascript. This makes templating only useful for application developers (and since Javascript can already achieve templating without the template tag, the `template` element kinda superfluous).

I propose to add a way to declaratively bind a template element to a custom element.

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https://github.com/w3c/webcomponents/issues/850