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Enable Typescript to understand JSX default props

See https://github.com/Microsoft/TypeScript/pull/24422

This addition allows a Component to use defaultProps without having to
declare them as optional, in a nutshell:

Before

class Before extends Component<{ prop?: string }> {
	static defaultProps = {
		prop: "default value"
	};

	render() {
		// this.props.prop is string|undefined
	}
}

const element = <Before />;

After

class After extends Component<{ prop: string }> {
	static defaultProps = {
		prop: "default value"
	};

	render() {
		// typeof this.props.prop is string
	}
}

const element = <After />;

The definition isn't perfect, it doesn't quite understand type unions where
the type of a single property changes, e.g.

{ type: "number"; value: number } | { type: "string"; value: string }

But this case doesn't break, it just would require you to still provide
a property.

There might be some more things we can do with LibraryManagedAttributes,
it could allow the children property to be correct within Components
(always an Array) whilst still allowing components to specify the type
of children they accept.

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