Add useMultiModal hook

[dawsonbooth]

Often, multiple modals are shown in sequence, serving as pages to one single conceptual modal. Rather than maintaining state that is updated with each modal action (often leading to the need for type assertions), we should have a hook that allows for a functional "bridge" between pages.

[dawsonbooth]

This can probably be abstracted to a more general useFlow hook, as the flow concept is not specific to modals