fix: add container to TypeScript types

[jason0x43]

Summary

Update the TS types for the new container property that was added to the output of render in #567.

Test plan

In a TS project using the current version of @testing-library/react-native (currently 7.1.0), try accessing container on the result of a render call. A typing error will occur because RenderAPI (the result of render) doesn't have a container property.

const bodyText = render(<BodyText>Foo</BodyText>);
expect(bodyText.container).toHaveTextContent('Foo'); // <-- will generate type error

With this PR, no error is shown.

[thymikee]

Thanks!