Accept DateTimeInterface in date_span instead of only DateTime

[javihgil]

Problem

date_span() currently type-hints DateTime.

That excludes DateTimeImmutable, even though immutable date objects are common in modern Symfony applications.

Current evidence

• src/Twig/Extension/DateSpanExtension.php accepts DateTime $dateTime
• the helper logic only needs a standard PHP date-time interface, not the mutable implementation specifically

Expected behavior

The filter should work with standard PHP date objects, including DateTimeImmutable.

Suggested work

• change the accepted type to DateTimeInterface
• add tests covering immutable date instances
• update documentation and feature definitions so the supported input types are explicit