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/GodotEnums.cs

@@ -2,6 +2,8 @@ using System;
 
 namespace Godot.SourceGenerators
 {
+    // TODO: May need to think about compatibility here. Could Godot change these values between minor versions?
+
     internal enum VariantType
     {
         Nil = 0,
@@ -131,4 +133,16 @@ namespace Godot.SourceGenerators
         DefaultIntl = 38,
         NoEditor = 2
     }
+
+    public enum MethodFlags
+    {
+        Normal = 1,
+        Editor = 2,
+        Const = 4,
+        Virtual = 8,
+        Vararg = 16,
+        Static = 32,
+        ObjectCore = 64,
+        Default = 1
+    }
 }