[next] Dispatch ready event for the custom py type (#1694)

pyscript.core/test/index.html

@@ -4,6 +4,9 @@
         <meta charset="UTF-8" />
         <meta name="viewport" content="width=device-width, initial-scale=1.0" />
         <title>PyScript Next</title>
+        <script>
+            addEventListener("py:ready", console.log);
+        </script>
         <link rel="stylesheet" href="../dist/core.css">
         <script type="module" src="../dist/core.js"></script>
     </head>