Enable support for typed children using TSX

[bitpshr]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

This PR adds the ElementChildrenAttribute interface inside the jsx namespace to enable typed children.

Resolves #675

[matt-gadd]

@bitpshr do you think we could get a typings test for this? might be worth building it off @agubler's PR #678 which fixes the TSX integration tests we'd neglected

[kitsonk]

Not 100% sure if it is relevant in this one, but we do have a rudimentary type assertions test tool over inter dojo/interfaces: https://github.com/dojo/interfaces/blob/master/tests/support/assertType.ts

[bitpshr]

@matt-gadd how could I test this properly? I have something like:

	'allows for typed children'() {
		class Bar extends WidgetBase { }
		class Foo extends WidgetBase<WidgetProperties, WNode<Bar>> {
			render() {
				return <Bar />;
			}
		}
	}

But since widget-core doesn't do runtime checks on children, using something like throws and adding an incorrect child inside Foo wouldn't actually throw. How can I test a compile-time type check?

[matt-gadd]

@bitpshr given #675, i'm going to go ahead and close this - sorry for wasting your time!