Simplify decodeBase64 polyfill if drop IE9?

As I understand this original part of `decodeBase64` polyfill was written 11 years ago:
https://github.com/emscripten-core/emscripten/blob/4a9d5ab94f66b629b99c1104663bedbc80f799c1/src/base64Utils.js#L11

Nowadays it could be simplify to this:
```js
var decodeBase64 = typeof window !== 'undefined' && typeof atob === 'function' ? atob : function (input) {
return Buffer.from(input, 'base64').toString('binary');
}
```
It always forces to use `Buffer` for node.js where `atob` didn't exist before v16.0.0 but still not recommend. For the rest platforms: all browsers and Deno `atob` present for a long time. Only exception is IE 6, 7, 8 and 9. And here the question is, are you still going to support IE9?