[IntelliSense] Add support to lit-element

[DavidBernal]

Hi, I noted the extension doesn't work with lit-element.

A lit element component looks like:

import { LitElement, html } from 'lit-element';

class MyElement extends LitElement {
  render() {
    return html`<p class="h-16">template content</p>`;
  }
}

Thank you