Add ChronosInterval class

[dereuromark]

Summary

This PR adds a ChronosInterval class that wraps DateInterval using the decorator pattern, as suggested by @markstory in #444.

Features

• ISO 8601 Duration Formatting: __toString() returns proper ISO 8601 format (e.g., P1Y2M3DT4H5M6S)
• Factory Methods:
  • create(string $spec) - Create from ISO 8601 spec
  • createFromValues(...) - Create from individual components
  • instance(DateInterval $interval) - Wrap an existing DateInterval
• Native Interop: toNative() returns the underlying DateInterval for compatibility
• Convenience Methods:
  • totalSeconds() - Approximate total seconds
  • totalDays() - Total days (exact if from diff())
  • isNegative() - Check if interval is inverted
  • isZero() - Check if interval has no duration
• Property Access: Proxies y, m, d, h, i, s, f, invert, days

Example Usage

// Create from spec
$interval = ChronosInterval::create('P1Y2M3D');
echo $interval; // "P1Y2M3D"

// Create from values
$interval = ChronosInterval::createFromValues(hours: 2, minutes: 30);
echo $interval; // "PT2H30M"

// Wrap existing DateInterval
$diff = $date1->diff($date2);
$interval = ChronosInterval::instance($diff);

// Use with native code
someFunctionExpectingDateInterval($interval->toNative());

Discussion Points

This is a draft PR to gather feedback on the API design:

1. Should we add arithmetic methods (add(), sub())?
2. Should ChronosInterval be returned from Chronos::diff() instead of DateInterval?
3. Any other methods that would be useful?

Related to #444

Test plan

• Unit tests for all public methods
• All existing tests pass (920 tests)
• Code style passes