Add asset open handler system and async open support

Introduced AssetOpenHandlerAttribute for extension-based asset open logic.
Implemented async OpenAssetAsync methods in IAssetRegistry and AssetRegistry, using reflection to invoke handlers.
Updated ContentBrowser and ViewModel to support async asset opening.
Refactored Hierarchy initialization and XAML for clarity.
Logger now throws exceptions in DEBUG for errors/asserts.
Removed unused code and cleaned up usings.

src/Editor/Ghost.Editor.Core/Attributes.cs

@@ -19,6 +19,19 @@ public class CustomEditorAttribute : DiscoverableAttributeBase
     }
 }
 
+public class AssetOpenHandlerAttribute : DiscoverableAttributeBase
+{
+    internal string[] Extensions
+    {
+        get;
+    }
+
+    public AssetOpenHandlerAttribute(params string[] extensions)
+    {
+        Extensions = extensions;
+    }
+}
+
 [AttributeUsage(AttributeTargets.Method)]
 public sealed class ContextMenuItemAttribute : DiscoverableAttributeBase
 {