feat(btoa): port and modify from Array.findindex

Gosh, I got confused by your code here. You reference `findindex` from `Array` (constructor), but actually it's a `prototype` method.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex

So you need to change it to:
```javascript
var ArrayPrototype = require("../helpers/ArrayPrototype");

module.exports = HAS_FINDINDEX ? ArrayPrototype.findIndex : function
```