[whatwg/dom] Change handling of is="" in element creation and cloning (#566)

Part of fixing https://github.com/whatwg/html/issues/3402. This changes two things:

* It fixes the cloning steps to consult the internal is value, instead of the is="" content attribute.
* It removes the step from createElement()/createElementNS() that would insert an is="" attribute in the created element, as we've now decided that is="" is a way of communicating to the parser, and not a general-purpose indicator of customized built-in-ness.

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This is coupled with a HTML change I will be submitting shortly, to the serializer (and to several examples).

@tkent-google, will you be able to help with tests for this? I am happy to review.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/566

-- Commit Summary --

  * Change handling of is="" in element creation and cloning

-- File Changes --

    M dom.bs (30)

-- Patch Links --

https://github.com/whatwg/dom/pull/566.patch
https://github.com/whatwg/dom/pull/566.diff

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