Add support for starting and stopping the tunnel

This commit introduces the Tunnel view model object which has
support for asking for permission, starting and stopping the
tunnel. It automatically updates its state and publishes
changes as an ObservableObject.

Apple/App/BurrowApp.swift

@@ -1,10 +1,18 @@
+import NetworkExtension
 import SwiftUI
 
 @main
+@MainActor
 struct BurrowApp: App {
+
+    static let tunnel = Tunnel { manager, proto in
+        proto.serverAddress = "hackclub.com"
+        manager.localizedDescription = "Burrow"
+    }
+
     var body: some Scene {
         WindowGroup {
-            ContentView()
+            TunnelView(tunnel: Self.tunnel)
         }
     }
 }