Open new browser window if no windows exist (#2610)

Task/Issue URL:
https://app.asana.com/0/1203137811378537/1207061568595472/f
Tech Design URL:
CC:

**Description**:

Fixes an issue when "Open DDG" doesn't create a new window if none
existing window is found.

**Steps to test this PR**:
1. With at least one DDG opened and in the background, open status bar
menu > Open DDG
2. Expect: DDG is brought to the foreground
3. Close all DDG windows
4. Open status bar menu > Open DDG
5. Expect: New DDG window is created

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###### Internal references:
[Pull Request Review
Checklist](https://app.asana.com/0/1202500774821704/1203764234894239/f)
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)
[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

DuckDuckGo/Application/URLEventHandler.swift

@@ -145,6 +145,8 @@ final class URLEventHandler {
             WindowControllersManager.shared.showPreferencesTab(withSelectedPane: .vpn)
         case AppLaunchCommand.shareFeedback.launchURL:
             WindowControllersManager.shared.showShareFeedbackModal()
+        case AppLaunchCommand.justOpen.launchURL:
+            WindowControllersManager.shared.showNewWindow()
         case AppLaunchCommand.showVPNLocations.launchURL:
             WindowControllersManager.shared.showPreferencesTab(withSelectedPane: .vpn)
             WindowControllersManager.shared.showLocationPickerSheet()

DuckDuckGo/Windows/View/WindowControllersManager.swift

@@ -237,6 +237,13 @@ extension WindowControllersManager {
         }
     }
 
+    func showNewWindow() {
+        guard WindowControllersManager.shared.lastKeyMainWindowController == nil else { return }
+        let tabCollection = TabCollection(tabs: [])
+        let tabCollectionViewModel = TabCollectionViewModel(tabCollection: tabCollection)
+        _ = WindowsManager.openNewWindow(with: tabCollectionViewModel)
+    }
+
     func showLocationPickerSheet() {
         let locationsViewController = VPNLocationsHostingViewController()
         let locationsWindowController = locationsViewController.wrappedInWindowController()