[WC] add support for custom web components with complex needs

[kyle-west]

This is the new interface for adding support for custom web components. The old way will still work, but I removed it from the docs.

new PersistentStateRegistry().registerCustomElement({
  // the tag name of your custom web component
  name: 'my-custom-input-element',
  
  // this is the property that <persistent-state> will initialize on your component with any stored values
  updateProperty: 'customValue',

  // this is the name of the event your component fires when it's internal input value changes
  changeEvent: 'my-custom-input-element::input-event-name',

  // This is a callback for the PersistentStateRegistry to manage changes from your element.
  // The return value from this callback will be what is stored/loaded from memory
  onChange: (customEvent) => {
    return customEvent.detail.customValue
  }
});