Support lazy widgets with a function that returns a promise in w()

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Supports lazy widgets directly in w() using a function that returns a Promise that resolves to a widget constructor.

class MyWidget extends WidgetBase {
    private _loadWidget = () => {
        return import('./lazy');
    }

    render() {
        return w(this._loadWidget, {});
    }
}

Note: The function cannot change across renders as it cannot be tracked.

Resolves #57