src/utils - Add getRootFiberNodeFromDOM helper

To find root Fiber node from overall html document or a particular element.
Ideally, this should be called with the same container element used in ReactDom.render.

src/mocked-types/index.ts

@@ -1,5 +1,13 @@
 import * as React from "react";
 
+export interface FiberNodeDOMContainer extends Element {
+  _reactRootContainer: {
+    _internalRoot: {
+      current: FiberNode | null;
+    };
+  };
+}
+
 export type FiberNode =
   | FiberNodeForComponentClass
   | FiberNodeForFunctionComponent