Use webpack to build JS for shortcode checkout

[reykjalin]

We currently use uglify to build the shortcode version of the required JavaScript for customers to checkout with Stripe. Since #1467 we now have a webpack-based build system set up and ready to go so we should take advantage of that and migrate the shortcode checkout JS to the webpack-based build system.

This will entail moving the js from assets/js to client/shortcode. As part of this we'll want to re-use and add API calls to the client/api module — the goal is to re-use as much code as possible between the blocks and shortcode versions of the JS.

[github-actions]

Hi,
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