Add useSyncExternalStore to react-debug-tools

Adds support for useSyncExternalStore to react-debug-tools, which in
turn adds support for React Devtools.

Test plan: I added a test to ReactHooksInspectionIntegration, based on
existing one for useMutableSource.

packages/react-debug-tools/src/ReactDebugHooks.js

@@ -269,7 +269,19 @@ function useSyncExternalStore<T>(
   subscribe: (() => void) => () => void,
   getSnapshot: () => T,
 ): T {
-  throw new Error('Not yet implemented');
+  // useSyncExternalStore() composes multiple hooks internally.
+  // Advance the current hook index the same number of times
+  // so that subsequent hooks have the right memoized state.
+  nextHook(); // SyncExternalStore
+  nextHook(); // LayoutEffect
+  nextHook(); // Effect
+  const value = getSnapshot();
+  hookLog.push({
+    primitive: 'SyncExternalStore',
+    stackError: new Error(),
+    value,
+  });
+  return value;
 }
 
 function useTransition(): [boolean, (() => void) => void] {

packages/react-debug-tools/src/__tests__/ReactHooksInspectionIntegration-test.js

@@ -878,4 +878,37 @@ describe('ReactHooksInspectionIntegration', () => {
       },
     ]);
   });
+
+  // @gate experimental || www
+  it('should support composite useSyncExternalStore hook', () => {
+    const useSyncExternalStore = React.unstable_useSyncExternalStore;
+    function Foo() {
+      const value = useSyncExternalStore(
+        () => () => {},
+        () => 'snapshot',
+      );
+      React.useMemo(() => 'memo', []);
+      return value;
+    }
+
+    const renderer = ReactTestRenderer.create(<Foo />);
+    const childFiber = renderer.root.findByType(Foo)._currentFiber();
+    const tree = ReactDebugTools.inspectHooksOfFiber(childFiber);
+    expect(tree).toEqual([
+      {
+        id: 0,
+        isStateEditable: false,
+        name: 'SyncExternalStore',
+        value: 'snapshot',
+        subHooks: [],
+      },
+      {
+        id: 1,
+        isStateEditable: false,
+        name: 'Memo',
+        value: 'memo',
+        subHooks: [],
+      },
+    ]);
+  });
 });