Fix reopening window on macOS (#5)

## Summary

This fixes an issue that caused macOS app rejection to AppStore.

> we found that when the user closes the main application window there
is no menu item to re-open it

When using `Window`, the "New window" menu item is not available.
Switching to `WindowGroup` solves the problem.

## What was done

- [x] Use `WindowGroup` instead of `Window` in macOS app
- [x] Remove unused `AppWindowID.main`

sort-swift-imports-app/Sources/AppFeature/App.swift

@@ -25,7 +25,7 @@ struct App: SwiftUI.App {
       )
     }
 #elseif os(macOS)
-    Window(appInfo.name, id: AppWindowID.main.rawValue) {
+    WindowGroup {
       AppView(
         store: Store(
           initialState: AppReducer.State(

sort-swift-imports-app/Sources/AppShared/AppWindowID.swift

@@ -1,6 +1,5 @@
 #if os(macOS)
 public enum AppWindowID: String {
-  case main = "main"
   case about = "about"
   case help = "help"
   case acknowledgments = "acknowledgments"