feat: add formatMultipleDates prop to DatePicker

[kykim00]

Description

Linked issue: #6066

Problem

When using selectsMultiple, the default display format shows:

• 1 date: 01/01/2024
• 2 dates: 01/01/2024, 01/15/2024
• 3+ dates: 01/01/2024 (+2)

Users have no way to customize this format without controlling the value prop directly.

Changes

This PR adds a new formatMultipleDates prop to customize how multiple selected dates are displayed in the input field when using selectsMultiple.

• src/index.tsx: Implement formatMultipleDates prop in getInputValue method
• src/test/multiple_selected_dates.test.tsx: Add tests for the new prop
• src/test/min_time_test.test.tsx: Fix type error caused by discriminated union update
• docs-site/: Add example for the new prop

Screenshots

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (928b2cf) to head (aa0af9d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6067   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          30       30           
  Lines        3590     3594    +4     
  Branches     1519     1521    +2     
=======================================
+ Hits         3562     3566    +4     
  Misses         27       27           
  Partials        1        1           

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