Add support for tns:MovingCondition in tns:IslamicHoliday

[XSpielinbox]

More information

Please enable to define child-elements of type tns:MovingCondition for elements of type tns:IslamicHoliday, as is possible with elements of type fixed.

This would make definition for e.g. the Hari Raya Haji and Hari Raya Puasa public holidays in Singapore (moved to the following Monday, if it would fall on a Sunday) way easier.

So, something like the following should be valid, what it is not at the moment:

<tns:IslamicHoliday type="ID_UL_ADHA_2" validFrom="2023" descriptionPropertiesKey="HARI_RAYA_HAJI">
    <tns:MovingCondition substitute="SUNDAY" with="NEXT" weekday="MONDAY"/>
</tns:IslamicHoliday>

see also https://www.mom.gov.sg/newsroom/press-replies/2008/rules-on-off-in-lieu

Currently when a public holiday falls on a Sunday, the following Monday will be, by law, a holiday.

[derTobsch]

see #283

[XSpielinbox]

Thank you!
I will update the Singapore holiday definitions accordingly as soon as possible.