feat: also publish a nuget package for c# scripting users

[jolexxa]

For C# users, it turns out calling native API's with the native Godot window handle works just fine from C# scripting. However, adding this project as a GDExtension to a C# scripting project results in warnings because of multiple .NET runtimes being present and other stuff with Godot. We'll still ship it as a GDExtension for GDScript users who may need this functionality, but we can also ship it as a standard nuget package for our C# scripting users.