Throw useful error if `atob` / `btoa` are not defined

`universal-github-app-jwt` is a universal library with different code for Node/Browser. It has a `"browser"` key to override the entry point for browser builds:

https://github.com/gr2m/universal-github-app-jwt/blob/ac358a2a93410881e3b7db7ebe8e10e2cb0c2121/package.json#L4-L6

Unfortunately build tools such as WebPack end up using this entry point, too, even when building for Node targets. See for example: https://github.com/octokit/auth-app.js/issues/59. There has been a lot of discussion around this topic, but no conclusion on how to resolve it.

But the least we could do is to check if `atob` exists in [src/utils.ts](https://github.com/gr2m/universal-github-app-jwt/blob/ac358a2a93410881e3b7db7ebe8e10e2cb0c2121/src/utils.ts). If it doesn't, then throw a helpful error which could include a link to the README file where we could add some information about this challenge.