[react-hooks] Provide proof-of-concept useUser hook

packages/@sanity/react-hooks/src/@types/parts.d.ts

@@ -1,2 +1,3 @@
 declare module 'part:@sanity/base/datastore/document'
 declare module 'part:@sanity/base/schema'
+declare module 'part:@sanity/base/user'

packages/@sanity/react-hooks/src/index.ts

@@ -5,3 +5,4 @@ export {useValidationStatus} from './useValidationStatus'
 export {useSyncState} from './useSyncState'
 export {useConnectionState} from './useConnectionState'
 export {useDocumentOperationEvent} from './useDocumentOperationEvent'
+export {useUser, LOADING_USER} from './useUser'

packages/@sanity/react-hooks/src/useUser.ts

@@ -0,0 +1,14 @@
+import userStore from 'part:@sanity/base/user'
+import {useObservable} from './utils/use-observable'
+
+export interface User {
+  id: string
+  displayName?: string
+  imageUrl?: string
+}
+
+export const LOADING_USER = Symbol.for('LOADING_USER')
+
+export function useUser(userId): User | null | typeof LOADING_USER {
+  return useObservable(userStore.observable.getUser(userId))
+}