ExportService gets removed and instead a global class registry is added.
The `init.js` file is split into `init_sync.js` and `init_async.js`
so that synchronous code that isn't dependent on imports is guarenteed
to run before initgui.js. The globalThis scope and service-script API
now expose `def`, a function for registering class definitions, and
`use`, a function for obtaining registered classes.
