[useSES shim] Import prefixed native API

Until useSyncExternalStore is finalized, the shim should import the
prefixed version (unstable_useSyncExternalStore), which is available
in the experimental builds. That way our early testers can start
using it.

Author: acdlite

packages/use-sync-external-store/src/useSyncExternalStore.js

@@ -17,7 +17,8 @@ const {
   useEffect,
   useLayoutEffect,
   useDebugValue,
-  useSyncExternalStore: builtInAPI,
+  // The built-in API is still prefixed.
+  unstable_useSyncExternalStore: builtInAPI,
 } = React;
 
 // Prefer the built-in API, if it exists. If it doesn't exist, then we assume