feat: add app settings manager to do the reading and writing

src/AltGen.Console/Program.cs

@@ -3,6 +3,7 @@
   .ConfigureServices(static (_, services) =>
   {
     services.AddSingleton<IFileSystem, FileSystem>();
+    services.AddSingleton<IAppSettingsManager, AppSettingsManager>();
     services.AddSingleton(AnsiConsole.Console);
     services.AddHttpClient<IAltGenService, AltGenService>()
       .AddStandardResilienceHandler();