Re: [WICG/webcomponents] The is="" attribute is confusing? Maybe we should encourage only ES6 class-based extension. (#509)

@andregs `has` requires inevitably a registry, so it'll mine the flexibility of the `ElementObserver`, but it's still possible.

```js
customBehaviors.define(
  'my-thing',
  {
    observedAttributes: ['some', 'attribute'],
    attributeChangedCallback(element, name, oldValue, newValue) {},
    connectedCallback(element) {},
    disconnectedCallback(element) {}
  }
);
```

The `customBehaviors` will verify that `my-thing` wasn't previously defined, and then it'll do something like this:

```js
const registry = new Map;
customBehaviors.define = (name, behavior) => {
  if (registry.has(name))
    throw new Error(`duplicated ${name}`);
  registry.set(name, {
    instance: new ElementObserver(behavior),
    options: {
      attributes: true,
      attributeFilter: behavior.observedAttributes || [],
      attributeOldValue: true
    }
  });
};

// a simulation of what should happen at the global level
const mo = new MutationObserver(records => {
  for (const {target, attributeName, oldValue} of records) {
    if (attributeName === 'has') {
      const newBehaviors = (target.getAttribute('has') || '').split(/\s+/);
      const oldBhaviors = (oldValue || '').split(/\s+/);
      for (const behavior of oldBhaviors) {
        if (customBehaviors.has(behavior) && !newBehaviors.includes(behavior))
          customBehaviors.get(behavior).instance.disconnect(target);
      }
      for (const behavior of newBehaviors) {
        if (customBehaviors.has(behavior) && !oldBhaviors.includes(behavior)) {
          const {instance, options} = customBehaviors.get(behavior);
          instance.observe(target, options);
        }
      }
    }
  }
});
mo.observe(document, {
  attributeFilter: ['has'],
  subtree: true
});
```

The only possible improvement I can see needed is an `eo.isObserving(target)` to avoid missing new defined behaviors already set as attribute.

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