Support Optional Typed Children

[agubler]

Type: bug

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:

Add support for using undefined as a union type to indicate that children are optional for the end user, and as such will possibly be undefined in the widget.

import { create } from '@dojo/framework/core/vdom';

type MyWidgetChildren = undefined | (() => string);

const factory = create().children<MyWidgetChildren>();

const MyWidget = factory(function MyWidget({ children }) {
    const [c] = children();
    if (c) {
        return c();
    }
    return null;
});

<MyWidget>{() => 'Hello Dojo'}</MyWidget>;
<MyWidget />;
w(MyWidget, {}, [() => 'Hello Dojo']);
w(MyWidget, {});
MyWidget({}, [() => 'Hello Dojo']);
MyWidget({});

Resolves issues with using w and calling functional widgets directly where the expected type was not being converted to an array but children is always expected to be an array.

Resolves #581