Run client-side JS through Babel

# Feature Request

## Motivation Behind Feature

This would allow us to just set the environments we want to support in the Babel config and stop worrying about which JS features are supported by which version of which WebView.

## Feature Description

As we have a build step anyways, so we would just have to add Babel to the build process.

The only real drawback I can see is that our generated `cordova.js` might not be as readable as before. But I'd be OK with that.

Real-life examples where not having this bit me: #203 #204

Begin with the existing build step and the generated cordova.js output; the issue does not name specific files or tests. Determine how the desired Babel environments should be configured, then verify the generated output and the real-life cases referenced in #203 and #204.

Written by the indexing model from the issue text.