🐞 Duplication of file selector window

### Description

This was an issue with welcome window (before the refactor), but it's still an issue when you disable the welcome window and don't have any files open.

### To Reproduce

Do this:
1. Disable & dismiss the welcome window.
2. Click on CodeEdit in the dock.
3. File selector should appear.

Now you can do this as many times as you want:
5. Click on CodeEdit in the dock (again).
6. Another file selector should appear.

### Expected behavior

The file selector should only open once,

and repeated clicks on the app should only focus whatever window is open.

### Version information

CodeEdit: 0.0.1-alpha (21)
macOS: 13.1 (22C65)
Xcode: 14.2 (14C18)

### Additional context

_No response_