dillon1000/react

Only expose FiberRoot at the top level instead of a Fiber (#8934)

It is strange to get a handle on the Fiber since it is one of possibly two
root Fibers. The correct way to get the current Fiber is through the
root.current.

I exposed the Fiber originally because I thought we might use it for
Portals but we went with a different approach. So we don't need this.