move `dayName` component to `Calendar.dayName` as a optional prop

[matthewbub]

There is a block of code in the src/react/Calendar/Calendar.tsx file that is used to display the days of week. e.g S, M, T, W, T, F, S this code block is currently hardcoded into the return statement of the Calendar component.

We should modify this bit to live as a customizable component that library users can define themselves. Same as the dayComponent prop we already have defined in this file.

This is the block that should be extracted from the return statement

// ...
<div style={{ display: 'flex' }}>
    {dayNames.map((dayName, idx) =>
      <div key={idx} style={{ display: 'inline-block', width: '14.28%', textAlign: 'center' }}>
        {dayName}
      </div>
    )}
</div>
// ...

Tasks

• [x] Create a new prop in the CalendarProps interface called dayNameComponent that allows users to customize the Day Names components
• [x] If the dayNameComponent prop is not provided by the user, assign the default code block shard above. ☝
• [x] If the dayNameComponent prop is provided by the user, map over the days of the week and render the custom dayNameComponent for each day.
• [x] (Optional) Create a new story in Storybook to test the happy/unhappy cases here.