[webcomponents] Custom element callbacks for builtins (#291)

I think in general web developers would appreciate having these callbacks available for normal elements too. See e.g., https://www.w3.org/Bugs/Public/show_bug.cgi?id=23250.

However, in that case they'd probably be in addition to what the browser already does internally for the element. That is not different from what `is=""` does though, so the proponents of that should be okay with this too.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/291