Enable moveBefore in experimental releases (facebook#32549)

Enabling feature detection of early DOM features in a framework is
reckless. I'm not judging other frameworks (but also a little bit).
Because if you do something like `if (moveBefore) moveBefore(a, b) else
insertBefore(a, b)` like we do and then the implementation has to change
there are still too many websites out there that it becomes impossible
to change it. It would break the web. It would instead have to change to
a different name. That's what happened with `contains` -> `includes`.
Counter to popular belief it didn't have anything to do with patching
prototypes. Therefore, ideally frameworks shouldn't start rely on it
until there's two implementations so that there's time for feedback.

That's why we didn't immediately enable this even in experimental.
However, at this point there's probably enough feature detection and it
has shipped long enough in Chrome that it's unlikely to be able to
change at this point.

We can enable it now. For now just in `@experimental` to see if we can
flush out issues with it before bringing it to stable.

Author: sebmarkbage

packages/shared/ReactFeatureFlags.js

@@ -193,7 +193,7 @@ export const disableLegacyContext = true;
 export const disableLegacyContextForFunctionComponents = true;
 
 // Enable the moveBefore() alternative to insertBefore(). This preserves states of moves.
-export const enableMoveBefore = false;
+export const enableMoveBefore = __EXPERIMENTAL__;
 
 // Disabled caching behavior of `react/cache` in client runtimes.
 export const disableClientCache = true;