Export ScopedMutator type

[joshkel]

It would be helpful to have access to the ScopedMutator, so that API helper functions can easily specify a useSWRConfig mutate function as a parameter. For example:

import { ScopedMutator, useSWRConfig } from 'swr';

async function myMultiStepApiTask(mutate: ScopedMutator) {
  await postToKey1();
  await mutate(key1);
  await postToKey2();
  await mutate(key2);
}

function MyMultiStepButton() {
  const { mutate } = useSWRConfig();

  return <button type="button" onClick={() => myMultiStepApiTask(mutate)}>Go</button>;
}

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