fix: fix TS type of useGracefulField

[RoystonS]

This is a fix for #3.

I saw two options for the fix:

1. Modify index.d.ts so it pulls the named export from useGracefulField
2. Modify useGracefulField.d.ts so it goes back to a default export instead of a named export.

I went with 2 for consistency because GracefulField.d.ts is still using a default export.