Add test template for testing set_custom_element_data on builtin custom elements

packages/svelte/tests/runtime-runes/samples/custom-element-attributes/main.svelte

@@ -15,6 +15,24 @@
 			}
 		});
 	}
+	if(!customElements.get('value-builtin')) {
+		customElements.define('value-builtin', class extends HTMLDivElement {
+
+			constructor() {
+				super();
+				this.attachShadow({ mode: 'open' });
+			}
+
+			set value(v) {
+				if (this.__value !== v) {
+					this.__value = v;
+					this.shadowRoot.innerHTML = `<span>${v}</span>`;
+				}
+			}
+		}, {
+			extends: "div"
+		});
+	}
 </script>
 
 <my-element string="test" object={{ test: true }}></my-element>
@@ -22,3 +40,4 @@
 
 <value-element value="test"></value-element>
 <value-element {...{value: "test"}}></value-element>
+<div is="value-builtin" value="test"></div>