[pugixml] Try COMPACT mode

[JanMarvin]

https://github.com/JanMarvin/openxlsx2/blob/7c94ede76e53f7e8c6a4a58ae5672887c6e91720/inst/include/pugixml/pugiconfig.hpp#L21

The xml DOM tree is known to cause memory issues. Especially with wider datasets. We can try to enable the PUGIXML_COMPACT mode which should reduce the memory usage 2-5 times according to the documentation. Though the doc also mentions performance costs and I assume there are reasons why it’s not enabled by default. So we should run some tests before causing issues.