Adds an import view that shows when the app is opening a .monster file.

MonsterCards/Views/ContentView.swift

@@ -10,6 +10,7 @@ import CoreData
 
 struct ContentView: View {
     @Environment(\.managedObjectContext) private var viewContext
+    @State private var isShowingImportDialog = false
  
     var body: some View {
         TabView {
@@ -35,6 +36,14 @@ struct ContentView: View {
                     Text("Library")
                 }
         }
+        .onOpenURL(perform: beginImportingMonster)
+        .sheet(isPresented: self.$isShowingImportDialog) {
+            Text("Importing Monster")
+        }
+    }
+    
+    func beginImportingMonster(url: URL) {
+        self.isShowingImportDialog = true
     }
 }