Fix TypeScript Typings Resolution for @pokt-foundation/pocketjs-signer

[luis-herasme]

If you install the library using the following command:

npm i @pokt-foundation/pocketjs-signer

And import it into your project like this:

import { KeyManager } from "@pokt-foundation/pocketjs-signer";

You will get the following error:

Could not find a declaration file for module '@pokt-foundation/pocketjs-signer'. '/Users/luis/github/new-wallet/node_modules/@pokt-foundation/pocketjs-signer/dist/index.js' implicitly has an 'any' type.
  There are types at '/Users/luis/github/new-wallet/node_modules/@pokt-foundation/pocketjs-signer/dist/src/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@pokt-foundation/pocketjs-signer' library may need to update its package.json or typings.

This pull request fixes the issue.