Add an optional positionCallback property to PopupSettings.

When using CSS transforms (e.g., transform: scale()) on container elements with dynamic values (CSS variables), the Popup component's positioning calculations don't account for the transformed coordinate space.

While the SCALE injection token exists, it only supports static values set at component initialization and cannot read dynamic CSS variables or respond to runtime changes.

• Enables custom positioning logic without breaking collision detection
• Supports dynamic transforms and CSS variable-based scaling
• Maintains backward compatibility (optional property)
• Eliminates the need for workarounds like MutationObserver
• Applicable to all components that use PopupSettings (DropDownList, ComboBox, MultiSelect, etc.)