[webcomponents] Move defineElement elsewhere? (#431)

Given the variety of proposals for new custom element related methods that are starting to fly around (e.g. #419 and #427 talk about upgrade-related helpers, plus there's always our plans for a registry API #154), we should start considering changes to the top-level API shape, and decide on something ASAP.

I can imagine three possibilities:

1. `document.customElements.define`. Everything goes on that object. `document.customElements` is null for documents without a browsing context.
2. `HTMLElement.define`. This makes it a bit clearer things are tied to the browsing context instead of the document. Other things can go there too. Right now the only things on there are a bunch of Node SHOUTY_CONSTANTS.
3. Punt on the problem: stick with `document.defineElement`, and just be sure to block future proposals on accepting one of the above two, at which point `document.defineElement` becomes an alias for one of those. This might not be feasible depending on how #419 goes.

@annevk @rniwa @travisleithead thoughts?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/431