Preparatory Work on Version 4

[CharliePoole]

@manfred-brands pointed out elsewhere that work is needed to make the version4 branch easier to work with before we start on any serious feature work.

This makes sense to me and I have identified a set of tasks, which seem to fall into the same category of "stuff" we should do first in order to make the codebase easier to work with...

1. Make V4 build using our NUnit.Cake.Recipe. This is done. The build is using the latest recipe version, 1.3.0.
2. Converting the code to nullable. V4 Codebase Should be Null-Enabled #1539.
3. Using internal visibility where appropriate. v4: Only permit integration through API methods exposed through nunit.engine.api #886
4. Port remaining relevant changes from V3. Port fixes from 3.18 releases #1547 Done

I believe that the first two items really must be done before we move further. We can be a bit more flexible on items 3 and 4.

Items 2 and 3 can either be done in separately or combined. If combined, I favor dealing with one assembly at a time, in bottom up fashion.

@nunit/engine-team I'd be happy to have volunteers for any part of this and I'll then keep the remaining tasks for myself.

[CharliePoole]

Porting of those V3 changes I identified as relevant is now complete. If I missed anything, I'm sure it will make itself known at some point.

[CharliePoole]

I just created a PR for the last change I feel is needed to make V4 ready for new features.

1. All V3 changes that apply to V4 are now ported.
2. The codebase is null-enabled.
3. There are separate nunit.common and nunit.extensibility assemblies
4. We have created nunt.agent.core and eliminated nunit.engine.core, separating the engine from the agents.

This opens the way to beginning work on actual major features and enhancements, in particular, replacing BinaryFormatter with a more secure alternative, converting all agents to pluggable extensions and adding a .NET 9.0 agent.