* call onDestroy when disconnected
* lifecycle hooks and custom elements
- Call onMount in connectedCallback for customElements
- register onMount return values as on_disconnect-callbacks for customElements
- run on_disconnect callbacks in disconnectedCallback
* do not reset on_mount so that it can fire again if reinserted
* simpler isCustomElement & skip extra function call
- pass options.customElement down to mount_component
- remove expensive isCustomElement check
- only call add_render_callback if not customElement
Co-authored-by: Pontus Lundin <pontus.lundin@ica.se>