Add ShadowRoot clonable attribute

Depends on D193677

Differential Revision: https://phabricator.services.mozilla.com/D195577

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1868428
gecko-commit: ade0be35500e4779e48c750433cdd6129471b049
gecko-reviewers: avandolder, webidl, emilio

shadow-dom/declarative/clonable.window.js

@@ -0,0 +1,10 @@
+test(() => {
+  const div = document.createElement("div");
+  const root = div.attachShadow({ mode: "open", clonable: true });
+  root.appendChild(document.createElement("input"));
+  assert_true(root.clonable, "clonable attribute");
+
+  const cloned = div.cloneNode(true);
+  assert_equals(cloned.shadowRoot.children.length, 1, "children count");
+  assert_equals(cloned.shadowRoot.children[0].localName, "input", "children content");
+}, "attachShadow with clonable: true");