Is the typescript definition exported correctly? #99

I'm having trouble with unfetch >= 3.1.1. The new TypeScript definition actually broke our build, and I'm not sure how to fix it.

The change was made in PR #89, in which the default export was removed from src/index.d.ts. Was this intentional? The PR is named "Fix isomorphic-unfetch definition" but packages/isomorphic-unfetch/index.d.ts is not changed, instead src/index.d.ts was changed.

Since src/index.mjs export default, shouldn't src/index.d.ts also export default?