[webcomponents] Rename the first argument of defineElement from type to localName (#416)

[`defineElement`](https://w3c.github.io/webcomponents/spec/custom/#dom-document-defineelement)'s first argument should be renamed to `localName`.

`type` is a very generic term and could refer to anything whereas `localName` in the context of DOM API is understood to refer to a [specific meaning](https://dom.spec.whatwg.org/#concept-element-local-name) and also matches the [definition of `createElement`](https://dom.spec.whatwg.org/#dom-document-createelement).

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/416