Initial work on a setup wizard

Setup wizard abstraction & architecture from TKMM

src/Ryujinx/UI/SetupWizard/Pages/SetupKeysPageViewModel.cs

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+using Avalonia.Controls;
+using Avalonia.Platform.Storage;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using Ryujinx.Ava;
+using Ryujinx.Ava.Common.Locale;
+using Ryujinx.Ava.UI.ViewModels;
+using System.Threading.Tasks;
+
+namespace Ryujinx.UI.SetupWizard.Pages
+{
+    public partial class SetupKeysPageViewModel : BaseModel
+    {
+        [ObservableProperty]
+        public partial string? KeysFolderPath { get; set; }
+
+        [RelayCommand]
+        private static async Task Browse(TextBox tb)
+        {
+            var result = await RyujinxApp.MainWindow.ViewModel.StorageProvider.OpenFolderPickerAsync(new FolderPickerOpenOptions {
+                Title = LocaleManager.Instance[LocaleKeys.SetupWizardKeysPageFolderPopupTitle],
+                AllowMultiple = false
+            }) switch {
+                [var target] => target.TryGetLocalPath(),
+                _ => null
+            };
+
+            if (result is not null) 
+            {
+                tb.Text = result;
+            }
+        }
+    }
+}