Implement a storybook util to detect when a story is scrolled into view

[mkrause]

We have a lot of stories where things happen automatically. For example, overlays like modals, dialogs, toast notifications, etc. This automatic triggering is needed because the UI must be visible without user interaction, for things like visual testing.

However for auto-generated docs pages (including all of the pages on the github.io storybook), this results in a bunch of things happening all at once, because all the stories are loaded on the page even if the user isn't actually viewing it.

We should investigate building a utility to only render a story if the user has scrolled to it. Something like the hidden="until-found" browser feature (but we can't use that yet due to browser support and this React issue).