[Dialog] support uncontrolled mode and `show()`/`close()` methods

[mayank99]

Changes

This extends the Dialog's ref using useImperativeHandle to add show()/close() methods.

Usage:

const dialog = React.useRef<typeof Dialog.Ref>(null);

<Button onClick={() => dialog.current?.show()}>Open</Button>
<Dialog ref={dialog}>…</Dialog>

Testing

Added unit test, updated stories.

Docs

Added docs.

[mayank99]

Superseded by #1983

The ref solution may feel more "react-y" but introduces difficulties:

• starts off as null because we cannot run custom code during initialization
• needs explicit typing using typeof Dialog.Ref
• you get a new ref every time the dialog mounts/unmounts from DOM.
• doesn't compose well. useImperativeHandle apparently needs to be defined to be defined for every single component, even for simple wrappers. (I couldn't even get the ref to work with Modal so far.)