Add typescript definitions.

[MicahZoltu]

When using this library in a TypeScript project, the lack of definitions makes it difficult to use. The addition of a .d.ts file that specifies the public API would resolve this.

[PeterTheOne]

What I used for the hash function only:

declare module 'eth-ens-namehash' {
  const namehash = {
    hash: (input: string) => string,
  };
  export default namehash;
}