Type declarations for playground #37

Hello:

I noticed the polyfill code in the playground directory — which appears to serve as documentation and as an opportunity for exploration. Is there any interest in including type declarations for users who'd like to explore in an environment outside a browser JS console? I was using it during a demo recently in VS Code and it would have been nice to have type information available.

I could create a PR if you'd like: 5b1ccf4...jsejcksn:proposal-arraybuffer-base64:feat/types