useCopyToClipboard: copy method not returning promise

The `copyToClipboard ` function is not returning a promise after copying text, see:

```
export default function App() {
  const [copiedText, copyToClipboard] = useCopyToClipboard();

  return (
    <button
      onClick={() => {
        copyToClipboard('text').then(() => {
          console.log('never gets here');
        });
      }}
    >
      copy
    </button>
  );
}
```