Make `ModalCurtain` initial focus configurable

[danoc]

The ModalBase component focuses on the first focusable element when opening but ModalCurtain focuses on the modal container. Focusing on the container is better for screen readers since users will be able to read the content of the modal from top to bottom.

Occasionally, it may make sense to skip directly to an input. Take this modal for example:

It's currently not possible to force ModalCurtain to focus on the input when the modal opens. Even if you grab the ref of the TextInput and call .focus(), the ModalCurtain will take away the focus as soon as its focus trap code runs.

I propose adding an initialFocus prop that will make this configurable.