Add selectCurrentUsername selector

Fixes #4724

Author: timdorr

docs/tutorials/essentials/part-4-using-data.md

@@ -1085,6 +1085,8 @@ const authSlice = createSlice({
 
 export const { userLoggedIn, userLoggedOut } = authSlice.actions
 
+export const selectCurrentUsername = (state: RootState) => state.auth.username
+
 export default authSlice.reducer
 ```