Start using the Assert helper class for checking parameters.

Start using the Assert helper class for checking parameters.

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