Implicit usings and file scoped namespaces

[lipchev]

Here's what I'm thinking:

• file scoped namespaces : nothing controversial here (I hope)
• implicit usings in UnitsNet - I'm thinking of System, System.Collections.Generic, System.Numerics (net8+) and our Units namespace
• implicit usings in the test projects: same as above + System.Linq and Xunit
• adding the JetBrains.Annotations package and annotating the tests with the [TestSubject(..)] annotation (not sure how useful this is in practice, but still better than nothing I guess).

[angularsen]

Everything except JetBrains.Annotations, just go for it. These are internal matters and we can YOLO a bit here.

JetBrains.Annotations I feel is not so much used anymore, it was mostly useful for nullability which is now supported by the framework.

An alternative to TestSubject is simply add xmldoc with <see cref="TheImplementation" /> and you'll have a nice clickable link to the class you're testing.

[lipchev]

I'm not 100% sure, but I suspect the AI might be using it (I noticed it was adding it when I use "Generate tests")- I still haven't tried it but my hope is that it would be able to "append" tests to the existing test files (directly using the context menu).