Add optional custom intersection root element in intersect plugin

[jannisborgers]

Intersection observer lets us provide an element as the intersection root. If I understand the plugin code correctly, the root is not explicitely set by the plugin, which lets the observer select the viewport as the intersection root.

Providing an optional root key in the options object and letting us pass an element to it (possibly via a new Alpine attribute on the container, or as a $ref?) would make this possible.