add getRootActiveElement utility function

packages/web-components/src/utils/root-active-element.ts

@@ -0,0 +1,10 @@
+// returns the active element in the shadow context of the element in question.
+export function getRootActiveElement(element: Element): Element | null {
+  const rootNode = element.getRootNode();
+
+  if (rootNode instanceof ShadowRoot) {
+    return rootNode.activeElement;
+  }
+
+  return document.activeElement;
+}