Support ES6 dynamic loading

[ELLIOTTCABLE]

Both JXA scripts and “Script Libraries” are evaluated as ES6 Scripts; this means, despite the JavaScriptCore in macOS 10.12 “Sierra” being modern enough to support¹ ES6 Modules, that there's no direct exposure of any method² to use them.

I'd like to write my JXA code as ES6 modules. There's obviously transpilers that will reduce ‘Module’ files (with export/import statements) to ‘script’ files (with require() calls) but I'd like to avoid this; what's needed, therefore, is a System.loader implementation that relies on the native tooling in JXA. I'm considering this as my next project.

Meanwhile, as jxa-async is presumably required for any such (obviously-asynchronous) Loader implementation, the jxa-async shim is presumably one of the very first modules I'd want to include in new JXA scripts. That, however, leaves a chicken-and-the-egg problem (can't have a Loader without asynchronicity; can't get asynchronicity using that Loader.)

There would have to be a combined shim that adds both jxa-async and a Loader to the current environment, something like:

Library('jxa-loader').shim(this)

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1. Specifically, ES6 modules can be written and loaded in HTML with the declarative <script type="module"> extension (see: Add <script type="module"> and module resolution/fetching/evaluation whatwg/html#443) as of Safari 10.1 … _↩︎

2. … but the corresponding programmatic API (await import(), as a function), accessible from within JavaScript — specifically within JavaScript Scripts — to consume ES6 Modules, has not yet landed. _↩︎