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

src/AltGen.Console/Config/AppSettingsManager.cs

@@ -0,0 +1,30 @@
+namespace AltGen.Console.Config;
+
+class AppSettingsManager(IFileSystem fileSystem) : IAppSettingsManager
+{
+  const string SettingsFileName = "appsettings.json";
+  readonly IFileSystem _fileSystem = fileSystem;
+
+  public bool AppSettingsExist()
+  {
+    return _fileSystem.Path.Exists(SettingsFileName);
+  }
+
+  public async Task<AppSettings> GetAppSettingsAsync()
+  {
+    var existingJson = await _fileSystem.File.ReadAllTextAsync(GetSettingsPath());
+    var existingAppSettings = JsonSerializer.Deserialize<AppSettings>(existingJson, JsonOptions.Default) ?? new AppSettings([]);
+    return existingAppSettings;
+  }
+
+  public async Task SaveAppSettingsAsync(AppSettings appSettings)
+  {
+    var json = JsonSerializer.Serialize(appSettings, JsonOptions.Default);
+    await _fileSystem.File.WriteAllTextAsync(GetSettingsPath(), json);
+  }
+
+  string GetSettingsPath()
+  {
+    return _fileSystem.Path.Combine(AppContext.BaseDirectory, SettingsFileName);
+  }
+}