Enable viewing file packages as folder on macOS (#21925)

The native `NSSavePanel` does not let you pick file packges like
`Application.app`-bundles by default.

Since the file picker does not return these bundles either,
picking these bundles is currently impossible.
By enabling `treatsFilePackagesAsDirectories` users can select
a bundle and pick it as a folder, and also select the contents of that
package.

native/Avalonia.Native/src/OSX/StorageProvider.mm

@@ -192,6 +192,7 @@ public:
             panel.canChooseDirectories = true;
             panel.canCreateDirectories = true;
             panel.canChooseFiles = false;
+            panel.treatsFilePackagesAsDirectories = true;
             
             if(title != nullptr)
             {