feat: allow function for dateFormat prop

[pataar]

Summary

• Extends the dateFormat prop to accept (date: Date) => string in addition to string | string[], enabling Intl.DateTimeFormat or any custom formatting logic
• Adds DateFormat type alias exported from the package for consumer use
• formatMultipleDates, safeDateRangeFormat, and safeMultipleDatesFormat are all compatible with function-based formats
• When dateFormat is a function, parsing typed input is gracefully skipped (no format string to parse against)
• Includes docs-site example demonstrating Intl.DateTimeFormat usage

Example usage

<DatePicker
  dateFormat={(date) =>
    new Intl.DateTimeFormat("en-US", {
      weekday: "short",
      year: "numeric",
      month: "long",
      day: "numeric",
    }).format(date)
  }
  selected={selectedDate}
  onChange={setSelectedDate}
/>

Test plan

• Unit tests for safeDateFormat, safeDateRangeFormat, safeMultipleDatesFormat with function dateFormat
• Integration tests for single date, range, partial range, and multiple dates with function dateFormat
• Integration test for formatMultipleDates callback with function dateFormat
• Full test suite passes (1499 tests)
• TypeScript type-check passes
• ESLint + Stylelint pass