feat: support ReactNode in ScrollVelocity texts prop

- Change texts: string[] to texts: React.ReactNode[] in TS variants
- Move   separator inside the span (encapsulated in scroller)
- Replace numCopies! non-null assertion with numCopies ?? 6
- Update prop table description in demo to reflect ReactNode support
- Applied consistently across all 4 variants (JS-CSS, JS-TW, TS-CSS, TS-TW)

This is a non-breaking change — plain strings are valid ReactNode values,
so all existing usage continues to work. The change unlocks rich content
like icons, styled spans, gradients, and mixed JSX in the scrolling ticker.