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/GodotMemberData.cs

@@ -22,6 +22,20 @@ namespace Godot.SourceGenerators
         public ITypeSymbol? RetSymbol { get; }
     }
 
+    public struct GodotSignalDelegateData
+    {
+        public GodotSignalDelegateData(string name, INamedTypeSymbol delegateSymbol, GodotMethodData invokeMethodData)
+        {
+            Name = name;
+            DelegateSymbol = delegateSymbol;
+            InvokeMethodData = invokeMethodData;
+        }
+
+        public string Name { get; }
+        public INamedTypeSymbol DelegateSymbol { get; }
+        public GodotMethodData InvokeMethodData { get; }
+    }
+
     public struct GodotPropertyData
     {
         public GodotPropertyData(IPropertySymbol propertySymbol, MarshalType type)