fix: add typings to package.json export

[kouloumos]

When importing the package in a project made with Vite, I get the following error:

Could not find a declaration file for module '@nostr-dev-kit/ndk-react'. '/my-dir/node_modules/@nostr-dev-kit/ndk-react/dist/ndk-react.es.js' implicitly has an 'any' type.
  There are types at '/my-dir/node_modules/@nostr-dev-kit/ndk-react/dist/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@nostr-dev-kit/ndk-react' library may need to update its package.json or typings.ts(7016)

This seems to be a known issue when using moduleResolution: "bundler" via Vite. You can see the related discussion in microsoft/TypeScript#52363 as well as the same fix applied to other projects [1, 2].

This PR adds the missing type specification in exports of package.json.

Steps to reproduce

1. Create a new vite-react-app using npx vite@latest .
2. Install the @nostr-dev-kit/ndk-react package and import it somewhere it in the application.
3. Confirm that the types are not able to load.