Initial shim of useSyncExternalStore

This sets up an initial shim implementation of useSyncExternalStore,
via the use-sync-external-store package. It's designed to mimic the
behavior of the built-in API, but is backwards compatible to any version
of React that supports hooks.

I have not yet implemented the built-in API, but once it exists, the
use-sync-external-store package will always prefer that one. Library
authors can depend on the shim and trust that their users get the
correct implementation.

See reactwg/react-18#86 for background
on the API.

The tests I've added here are designed to run against both the shim and
built-in implementation, using our variant test flag feature. Tests that
only apply to concurrent roots will live in a separate suite.

packages/use-sync-external-store/src/__tests__/useSyncExternalStore-test.js → packages/use-sync-external-store/extra.js

@@ -4,11 +4,9 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @emails react-core
+ * @flow
  */
 
 'use strict';
 
-describe('useSyncExternalStore', () => {
-  test('TODO', () => {});
-});
+export * from './src/useSyncExternalStoreExtra';

packages/use-sync-external-store/npm/extra.js

@@ -0,0 +1,7 @@
+'use strict';
+
+if (process.env.NODE_ENV === 'production') {
+  module.exports = require('./cjs/use-sync-external-store-extra.production.min.js');
+} else {
+  module.exports = require('./cjs/use-sync-external-store-extra.development.js');
+}

packages/use-sync-external-store/package.json

@@ -12,6 +12,7 @@
     "README.md",
     "build-info.json",
     "index.js",
+    "extra.js",
     "cjs/"
   ],
   "license": "MIT",