add unselect option for touchbarscrubber

[Lynoma]

This PR intends to add an option to unselect the scrubbar item after being selected. This allows the Scrubber to be used as a container for buttons to be clicked instead of selected.

To use it you just have to set the shouldUnselectAfterHit to true.

TouchBarScrubber(
        children: scrubberChildren,
        selectedStyle: ScrubberSelectionStyle.roundedBackground,
        overlayStyle: ScrubberSelectionStyle.outlineOverlay,
        mode: ScrubberMode.fixed,
        shouldUnselectAfterHit: true,
        onSelect: (childId) {
          
        },
        onHighlight: (childId) {

        },
)

Thanks for taking the time to review it 🙏

[vital-edu]

Thanks @Lynoma for the contribution.

After reviewing your code I decided to modify it to eliminate a bug that was occurring:
if you keep pressing a scrubber and move it to another item, the item was not being unselected.