feat: update counter example

examples/demo/Counter.mjs

@@ -1,14 +1,16 @@
 // @ts-check
-import { WebComponent } from "../../src/index.js"
+import { WebComponent, html } from "../../src/index.js"
 
 export class Counter extends WebComponent {
-  static properties = ["count"];
-  onInit() {
-    this.props.count = 0;
-    this.onclick = () => ++this.props.count;
+  static props = {
+    count: 0
   }
   get template() {
-    return `<button id="btn">${this.props.count}</button>`;
+    return html`
+      <button onClick=${() => ++this.props.count} id="btn">
+        ${this.props.count}
+      </button>
+    `;
   }
 }