[w3c/webcomponents] Add global reflecting attributes to ElementInternals? (#821)

Since reflecting IDL attributes create HTML attributes on an element when set, and there may be cases where that's undesired, or even disallowed (setting attributes in a constructor) - should some or all of the global attributes have properties added to ElementInternals? And should global attributes going forward be added to ElementInternals?

A use case would be an element that wants to perform some initialize a value in a custom element constructor

```js
class MyElement extends HTMLElement {
  constructor() {
    this.title = 'A Custom Element';
  }
}
```

This will report an error because the `title` attribute is created. I've seen people hit this with `title` and recently with `ariaPressed` in Safari. `ariaPressed` will be handled by ElementInternals, so should other attributes be to?

Looking at the current list of global attributes, including `title` (but excluding `tabindex`, which might be covered differently like in #762) I could possibly see arguments that they're not actually internal and _should_ only be set externally, ie an element shouldn't sprout it's own tooltip, but I'm not sure if or how that's been rationalized yet.

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