Simplistic Calculator (#1030)

* Simplistic Calculator, initial commit

* Add Linux runner

* Add macOS runner, update `main.dart` for iOS

* Add a test

simplistic_calculator/macos/Runner/AppDelegate.swift

@@ -0,0 +1,9 @@
+import Cocoa
+import FlutterMacOS
+
+@NSApplicationMain
+class AppDelegate: FlutterAppDelegate {
+  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return true
+  }
+}