Called when the host is connected to the component tree. For custom
element hosts, this corresponds to the connectedCallback() lifecycle,
which is only called when the component is connected to the document.
Called when the host is disconnected from the component tree. For custom
element hosts, this corresponds to the disconnectedCallback() lifecycle,
which is called the host or an ancestor component is disconnected from the
document.
A ReactiveController which tracks whether a slot is populated.