Add [BindSharedEnum] attribute to share namespace-level enums across GDExtension classes

[MLAcookie]

Summary

This PR introduces [BindSharedEnum] — a new attribute that allows namespace-level enums to be shared across multiple GDExtension [GodotClass] classes. Before, enums registered via [BindEnum] must be nested inside a [GodotClass] because the source generator only scans class members, forcing duplication when multiple extension classes need the same enum.

Before (enum duplicated per class)

// Each class must declare and register its own copy of the same enum
[GodotClass]
public partial class ClassA : Node
{
    [BindEnum]
    public enum Team { Red, Green, Blue }
}

[GodotClass]
public partial class ClassB : Node
{
    [BindEnum]
    public enum Team { Red, Green, Blue }  // Duplicated!
}

After (enum declared once, shared)

// Declare once at namespace level
public enum Team { Red, Green, Blue }

[GodotClass]
[BindSharedEnum(typeof(Team))]
public partial class ClassA : Node { }

[GodotClass]
[BindSharedEnum(typeof(Team))]
public partial class ClassB : Node { }

// sharing multiple enums on a single class
[GodotClass]
[BindSharedEnum(typeof(Team))]
[BindSharedEnum(typeof(ItemFlags))]
[BindSharedEnum(typeof(Status))]
public partial class Player : Node2D { }

Compatibility

• Existing [BindEnum] on nested enums continues to work unchanged.
• [BindSharedEnum] only affects classes that explicitly use it.
• No breaking changes to the public API surface of Godot.Bindings.

Note

I recognize that godot-dotnet may not currently be in a phase where convenience features like this are a priority, and the [BindEnum] pattern already covers the basic use case. If this doesn't align with the current roadmap, feel free to close without hesitation.

Either way, thank you for your time reviewing this.