Add capability to style the currently selected time unit

[QuenK30]

This PR introduces the ability to customize the style of the currently selected item (hour, minute, second, or day) in the
TimerPicker. Previously, all items shared the same styling, making it difficult to visually distinguish the exact selected value beyond its central position.

Key Changes

Styling System: Added a new selectedPickerItem property to
CustomTimerPickerStyles in styles.ts

Logic Flow: Updated TimerPicker.tsx to track and pass the current selected state down to the individual scroll components.

Component Update: Modified DurationScroll.tsx and PickerItem.tsx to receive the selectedValue and apply the selectedPickerItem style conditionally when an item is selected.

How to use
Users can now pass a selectedPickerItem object within the styles prop to highlight the active selection:

<TimerPicker
    styles={{
        selectedPickerItem: {
            color: '#FFFFFF',
            fontWeight: 'bold',
            fontSize: 28,
        },
    }}
/>

Verification
Automated tests: All 275 tests passed successfully (npm test).
Manual verification: Confirmed that the style is correctly applied to the center item and updates during scrolling.

This is my first PR for an open-source project, so thank you for your patience; I’m happy to make any changes or improvements based on your feedback!

[troberts-28]

Hey @QuenK30, thank you for putting this PR together! Will review ASAP and get back to you 🙌

[troberts-28]

Hey @QuenK30 👋 This looks great - thank you! The only thing missing is a corresponding update to the README. Will get this merged as soon as you've added that

[troberts-28]

Would marginally prefer isSelected && styles.selectedPickerItem here

[QuenK30]

I hope I didn't go off topic with this little change