Update custom constraints examples to include static extension members

[stevenaw]

The documentation for writing custom constraints includes some guidance to extend NUnit's Is class with a custom one in order expose a member on it to return the custom constraint.

https://docs.nunit.org/articles/nunit/extending-nunit/Custom-Constraints.html

C# 14 introduces extension members, including static methods, would could be a simpler option for some.