Fix createFromFormat to respect setTestNow for missing components

[dereuromark]

Summary

When setTestNow() is used for testing, createFromFormat() now uses values from the test time for any date/time components not present in the format string.

Previously, partial formats like createFromFormat('m-d', '10-05') would use the real system time for the year, making tests unreliable. Now it uses the testNow year.

The fix:

• Parses the format string to detect which components (year, month, day, hour, minute, second, microsecond) are present
• For missing components, uses values from testNow instead of real current time
• Properly handles escaped characters in format strings
• Respects ! and | modifiers which intentionally reset unspecified components to Unix epoch/zero

Example

Chronos::setTestNow(new Chronos('2020-12-01 00:00:00'));
$date = Chronos::createFromFormat('m-d', '10-05');
// Before: year from real system time (unpredictable in tests)
// After: 2020-10-05 00:00:00 (year from testNow)

Test plan

• Added tests for various partial format scenarios
• Added tests for escaped characters handling
• Added tests for ! and | reset modifiers
• All existing tests pass (909 tests)
• Code style passes

Closes #452

[markstory]

Why are we adding new syntax to PHP's datetime formats?

[dereuromark]

We're not adding new syntax - ! and | are existing PHP DateTime format modifiers (see PHP docs).

The code is detecting their presence so we know to skip applying testNow values. When a user explicitly uses ! or |, they're telling PHP to reset unspecified components to Unix epoch, so we should respect that intent rather than substituting testNow values.