Support nullable annotations

[jonasdoerr]

When I started using FlaUI, I realized that methods and properties I didn't expect to return null. So I thought nullable annotations could be useful.

• Enabled nullable annotations for Core/UIA2/UIA3, upgraded to C# 11 for nameof(parameter).
• Created internal fake code analysis attributes for TFMs not providing them, e.g. .NET Framework and .NET Standard 2.0.
• Nearly no changes in behaviour, only in a few cases where avoiding a null-ref exception was reasonable.
• Of course, there are still many chances for null-ref in the codebase, but we have warnings now.

[jonasdoerr]

The error in the build pipeline is likely to be caused by the version of the NUnit engine. Some versions had that issue that they throw an exception if they find an installed .NET runtime they don't know. Therefore, you can break a pipeline by just installing the .NET 8 runtime or SDK, without touching the build script. Propably, updating the NUnit console runner could fix it.