[CmdPal] Single settings window (#38069)

## Summary of the Pull Request

Make settings window single.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] **Closes:** https://github.com/zadjii-msft/PowerToys/issues/581

## Validation Steps Performed

Manual test.
- Open settings windows multiple times
- Activate minimized settings window

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Messages/SettingsWindowClosedMessage.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CmdPal.UI.ViewModels.Messages;
+
+public record SettingsWindowClosedMessage
+{
+}