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/PropertyInfo.cs

@@ -0,0 +1,23 @@
+namespace Godot.SourceGenerators
+{
+    internal struct PropertyInfo
+    {
+        public PropertyInfo(VariantType type, string name, PropertyHint hint,
+            string? hintString, PropertyUsageFlags usage, bool exported)
+        {
+            Type = type;
+            Name = name;
+            Hint = hint;
+            HintString = hintString;
+            Usage = usage;
+            Exported = exported;
+        }
+
+        public VariantType Type { get; }
+        public string Name { get; }
+        public PropertyHint Hint { get; }
+        public string? HintString { get; }
+        public PropertyUsageFlags Usage { get; }
+        public bool Exported { get; }
+    }
+}