C#: Add source generator for signals as events

Changed the signal declaration signal to:

```
// The following generates a MySignal event
[Signal] public delegate void MySignalEventHandler(int param);
```

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/GodotClasses.cs

@@ -5,6 +5,7 @@ namespace Godot.SourceGenerators
         public const string Object = "Godot.Object";
         public const string AssemblyHasScriptsAttr = "Godot.AssemblyHasScriptsAttribute";
         public const string ExportAttr = "Godot.ExportAttribute";
+        public const string SignalAttr = "Godot.SignalAttribute";
         public const string GodotClassNameAttr = "Godot.GodotClassName";
         public const string SystemFlagsAttr = "System.FlagsAttribute";
     }