Expose internal value for isServer

[Guria]

It might be useful across the app to reuse alreday calculated value:

import { env } from "~/env"

  if (env.isServer) {
    // Server: always make a new query client
    return createQueryClient();
  }
  // Browser: use singleton pattern to keep the same query client
  return (clientQueryClientSingleton ??= createQueryClient());