[webcomponents] s/document.registerElement/document.defineElement and no return value (#400)

Addresses #365
The spec still talks about element registration, registry of element definitions and has the type ElementRegistrationOptions but I think we can keep those.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/400

-- Commit Summary --

  * s/document.registerElement/document.defineElement and no return value

-- File Changes --

    M spec/custom/custom-elements-whole-world.svg (4)
    M spec/custom/index.html (18)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/400.patch
https://github.com/w3c/webcomponents/pull/400.diff

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https://github.com/w3c/webcomponents/pull/400