fix useStaggerScroll flexibility

[oEnzoRibas]

useStaggerScroll hardcodes its ScrollTrigger options (start, toggleActions) and doesn’t allow callers to override them, unlike useScrollTrigger/useScrollTimeline which accept scrollVars. Consider adding an optional scrollVars?: Partial<ScrollTrigger.Vars> parameter and spreading it into the ScrollTrigger config for consistency and flexibility.

Originally posted by @Copilot in #73