Add usePageVisibilityEffect and deprecate useVisibilityEvent #541

useVisibilityEvent runs a callback when the value of usePageVisibility would change, but the names do not show that the two are a pair. It is also the only hook with an *Event suffix, while useOutsideClickEffect uses *Effect for the same shape.

Add usePageVisibilityEffect with the same signature and behavior, mark useVisibilityEvent as deprecated with a pointer to the new name, and switch internal usages such as useImpressionRef to the new hook. Update docs and tests.