feat: subscription management in effect sys

[harrysolovay]

For instance...

let i = 0;
const subscription = rpcSubscription(config, "chain_subscribeNewHead", [], (close) => {
  return (m) => {
    i++;
    if (i > 5) {
      close();
    }
    console.log(m);
  };
});
U.throwIfError(await run(subscription));