feat: Add renderCustomDayName prop for custom weekday header rendering

[gnato]

Description

Adds renderCustomDayName prop to DatePicker component, enabling custom rendering of weekday names in the calendar header (similar to existing renderCustomHeader functionality).

Motivation

Provides developers with flexibility to customize day name display, such as adding emojis, icons, or custom styling based on the displayed month position when showing multiple months.

Changes

• Added ReactDatePickerCustomDayNameProps interface
• Added renderCustomDayName prop to Calendar component
• Added customDayNameCount parameter (similar to customHeaderCount)
• Added 7 comprehensive test cases
• Added example in docs-site
• Exported new interface in public API

Example Usage

<DatePicker
  monthsShown={2}
  renderCustomDayName={({ shortName, fullName, customDayNameCount }) => (
    <span style={{ fontWeight: customDayNameCount === 0 ? 'bold' : 'normal' }}>
      {shortName}
    </span>
  )}
/>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (b198c49) to head (33b0fa2).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6091      +/-   ##
==========================================
+ Coverage   99.14%   99.22%   +0.08%     
==========================================
  Files          30       30              
  Lines        3612     3620       +8     
  Branches     1548     1535      -13     
==========================================
+ Hits         3581     3592      +11     
+ Misses         29       27       -2     
+ Partials        2        1       -1     

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