C#: Add a Roslyn analyzer for global classes

[398utubzyt]

Should help clarify things to those who aren't as familiar with the core engine. This helps prevent one of many likely scenarios of the following from successfully compiling but not showing up in the editor (or even causing future crashes):

using Godot;

[GlobalClass]
public partial class ExampleGlobalGenericBad<T> : ExampleGlobalNode
{
    public override void _Ready()
    {
        GD.Print($"Woohoo, {GetType()}");
    }
}

[raulsntos]

Thanks for contributing to the .NET module! This is looking great, I think it can be very helpful to have an analyzer for this.

Adding errors where there weren't before breaks source compat. But the analyzer adds errors to code that used to be invalid or otherwise would not work as expected, so I think we're OK with the breaking change.

Also, since you are a new contributor, make sure to read CONTRIBUTING.md and the contributing documentation if you haven't already.

You'll need to squash the commits before this PR can be merged. The contributing documentation contains information about squashing in case you need it.

Feel free to reach out in the development chat if you need help.

[raulsntos]

This already looks great. I only added a few comments about the diagnostic messages to try and be consistent with existing diagnostic messages.

[raulsntos]

LGTM. Thank you very much!

[YuriSizov]

Thanks! And congrats on your first merged Godot PR, I believe!