Add a sort and reorder ranking widget

[AlexSkrypnyk]

Summary

Add a widget that lets the user reorder a list to express a ranking, returning the items in their final order.

Details

The multiselect widget chooses a subset but cannot capture order.

Add a sort/reorder widget: the user moves the highlighted item up or down to arrange the list, and the widget returns the items in their final order. Useful wherever sequence matters (priority lists, step ordering, preference ranking).

Acceptance criteria

• A new widget lets the user move the highlighted item up or down and returns the final order.
• The widget participates in the fluent builder and honours default (the initial order).
• Headless (JSON/env) collection accepts an ordered list and the JSON schema describes it.
• Rendering is theme-driven and works in Unicode and ASCII modes.
• Unit tests cover reordering and headless parity.
• The widget is documented in the README widgets section.