[Sentinel] Add support for boolean events

[titouanmathis]

As of now, we need to listend to the intersected event when using the Sentinel logic:

import { Sentinel } from '@studiometa/ui';

class App extends Base {
  static config = {
    name: 'App',
    components: { 
      Sentinel 
    },
  };

  onSentinelIntersected(entries) {
    if (entries[0].isIntersecting) {
      // do something when the sentinel is visible
    }
  }
}

It would be simpler to listen to dedicated events for both when the sentinel is intersecting and it is not.

  onSentinelVisible(entries) {}
  onSentinelHidden(entries) {}