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

Although I agree type is not a great word, I think it's important to distinguish from localName, because of the type extensions/ { is } version.

Assuming #408, then if you do

```js
document.defineElement("super-a", XFoo, { extends: "a" });

document.createElement("a", { is: "super-a" });
```

then the resulting local name of such elements will be `"a"`, not `"super-a"`.

I also think it's important to have a term for what is currently "valid custom element type".

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