Detect ES6 support in loader & fetch corresponding CDN bundle

[mydea]

Currently, the Loader Script will always fetch the ES5 bundle, unless replay is enabled, then we always fetch the ES6 bundle (because replay does not support ES5).

This has two negative side effects:

• When replay is not enabled, the majority of users that actually have an ES6 browser will get the less optimized ES5 version shipped
• When replay is enabled, users with ES5 browsers will not get a working sentry instance.

I propose to detect ES6 support (e.g. via checking <script nomodule>, which should be good enough for our purposes), and then loading the ES6 bundles based on this. If replay is enabled and there is no ES6 support, we disable replay and load the replay-less ES5 bundle.