Add selectionWrapper prop for customizable selection styling

[LeeSeongJinCa]

Problem:
The current react-big-calendar does not allow developers to customize the appearance of the selection overlay that appears when dragging to select cells.

Solution:

• Added a new selectionWrapper prop under the components option.
• selectionWrapper accepts a React component that wraps the default selection overlay, allowing full control over its rendering and styles.
• Updated stories/Calendar.stories.js with a CustomSelection example demonstrating how to use selectionWrapper to apply custom background colors, borders, shadows, etc.

Usage Example:

import CustomSelectionWrapper from './CustomSelectionWrapper';

<Calendar
  components={{
    selectionWrapper: CustomSelectionWrapper
  }}
  /* ...other props... */
/>

[LeeSeongJinCa]

@cutterbl Hello there :) ,

Could you take a look at this PR when you have a sec?
Would love your thoughts.

Thanks!

[cutterbl]

@LeeSeongJinCa There are some conflicts that must be resolved, prior to bringing this in.