Add vpu-lit-element back, used by too many things still

packages/common/vpu-lit-element.js

+import {LitElement} from "lit-element";
+
+export default class VPULitElement extends LitElement {
+    _(selector) {
+        return this.shadowRoot === null ? this.querySelector(selector) : this.shadowRoot.querySelector(selector);
+    }
+}