cc-form-control-element: add form getter

[pdesoyres-cc]

As stated in MDN documentation: https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/form, native input elements have a readonly property form that returns the associated <form> element (if any).

We can do the same for our control elements. Since CcFormControlElement uses the ElementInternals API, it's very easy to rely on this._elementInternal inner property to return the associated <form> element.