Enhancement

In order to support automatic code splitting for higher order components such as Outlet, it would be useful to support a function that returns a promise of a widget when using w or perhaps an additional API (so not to unnecessarily expose the API to the end user).

Within WidgetBase this function would be registered against an id and added to the registry, the function would then be replaced by the id on subsequent renders and the rest of the processing would flow through the usual registry path.

Note: it would only be able to support functions that don't change across renders so the function could not be defined inline, such as () => imports('./mywidget')