refactor: Add pubspec in dart example (#908)

Author: mbfakourii

packages/dart/example/main.dart

@@ -1,4 +1,4 @@
-import '../lib/parse_server_sdk.dart';
+import 'package:parse_server_sdk/parse_server_sdk.dart';
 
 Future<void> main() async {
   // Parse initialize

packages/dart/example/pubspec.yaml

@@ -0,0 +1,14 @@
+name: example
+description: example
+version: 1.0.0
+publish_to: 'none'
+
+environment:
+  sdk: ">=2.18.0 <4.0.0"
+
+dependencies:
+  parse_server_sdk:
+    path: ../
+
+dev_dependencies:
+  lints: ^1.0.1