added devcontainer with all tools for frontent (deno, nextjs) and backend (dart, go)

backend-dart/test/backend_dart_test.dart

@@ -0,0 +1,8 @@
+import 'package:backend_dart/backend_dart.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('calculate', () {
+    expect(calculate(), 42);
+  });
+}