constructor() {
    super();
    /**
     * @override FormRegistrarMixin
     * @protected
     */
    this._isFormOrFieldset = true;
    /**
     * @type {'child' | 'choice-group' | 'fieldset'}
     * @override FormControlMixin
     * @protected
     */
    this._repropagationRole = 'fieldset';
  }
@desc LionFieldset is basically a 'sub form' and can have its own nested sub forms.
It mimics the native <fieldset> element in this sense, but has all the functionality of
a FormControl (advanced styling, validation, interaction states etc.) Also see
FormGroupMixin it depends on.

LionFieldset enables the '_isFormOrFieldset' flag in FormRegistrarMixin. This makes .formElements
act not only as an array, but also as an object (see FormRegistarMixin for more information).
As a bonus, It can also group children having names ending with '[]'.

Above will be  helpful for both forms and sub forms, which can contain sub forms as children
as well and allow for a nested form structure.
Contrary, other form groups (choice groups like radio-group, checkbox-group and (multi)select)
don't: they should be considered 'end nodes' or 'leaves' of the form and their children/formElements
cannot be accessed individually via object keys.

@customElement lion-fieldset