Bump phpstan analysis level from 3 to 4

[slayerfx]

• Broaden PHPDoc parameter types for setter methods accepting flexible inputs (int → int|string|null in DocumentInformations, DocumentProperties)
• Broaden PHPDoc return types for nullable getters in Task (getDuration, getStartDate, getEndDate, getProgress — added |null)
• Fix Reader/MsProjectMPX::$iParentTaskIdx PHPDoc (was integer, now integer|null)
• Bump phpstan analysis level from 3 to 4