Store instance handles in an internal map behind flag (#35053)

We already append `randomKey` to each handle name to prevent external
libraries from accessing and relying on these internals. But more
libraries recently have been getting around this by simply iterating
over the element properties and using a `startsWith` check.

This flag allows us to experiment with moving these handles to an
internal map.

This PR starts with the two most common internals, the props object and
the fiber. We can consider moving additional properties such as the
container root and others depending on perf results.