This introduces https://github.com/wmde/Assert as a dependency,
as discussed in the RFC T91071.
This change uses assertions to check some parameters in some places,
to showcase the main intended use case for assertions in MediaWiki.
Bug: T91071
Change-Id: I93ac39b7c146f10532e37b51d973b59b9c424b2f
Swapped some "$var type" to "type $var" or added missing types
before the $var. Changed some other types to match the more common
spelling. Makes beginning of some text in captial.
Also added some missing @param.
Change-Id: Ic8aaf0a93796b97d0fa4617c1f86ff59f4b36131
