fix(form-core): remove usage of Public Class Fields to not break builds

.changeset/smooth-apricots-laugh.md

@@ -0,0 +1,5 @@
+---
+'@lion/form-core': patch
+---
+
+Remove usage of Public Class Fields to not break builds

packages/babel-plugin-extend-docs/demo/src/MyCounter.js

@@ -1,9 +1,11 @@
 import { LitElement, html, css } from 'lit-element';
 
 export class MyCounter extends LitElement {
-  static properties = {
+  static get properties() {
+    return {
       count: { type: Number },
     };
+  }
 
   static styles = css`
     :host {

packages/form-core/src/registration/FormRegisteringMixin.js

@@ -16,8 +16,11 @@ import { dedupeMixin } from '@lion/core';
  */
 const FormRegisteringMixinImplementation = superclass =>
   class extends superclass {
+    constructor() {
+      super();
       /** @type {FormRegistrarHost | undefined} */
-    __parentFormGroup;
+      this.__parentFormGroup = undefined;
+    }
 
     connectedCallback() {
       // @ts-expect-error check it anyway, because could be lit-element extension