Implement fetch API

[bghgary]

When we do this, we can update Babylon.js to use the fetch API instead of XMLHttpRequest. Perhaps even deprecate XMLHttpRequest in this repo.

Note that fetch polyfill with XMLHttpRequest does work.

[matthargett]

Deprecating the old and replacing with modern API has my vote. Getting bundle sizes down, and eliminating mixed uses in client code would be the primary forces IMO